You could create a folder below your TeX home directory and put your .sty file therein. Use this command at the command prompt to find out where:

kpsewhich -var-value=TEXMFHOME

On my computer it shows

C:/Users/stefan/texmf

but it might also be ~/texmf/ on a Linux or Unix computer.

Following the TeX directory structure, you should place your file in a subdirectory like ~/texmf/tex/latex/commonstuff/, according to Arthur's comment below. This has the advantage that it is not necessary to update the package database as TeX searches your personal texmf tree directly. If there is an ls-R file in your home texmf tree you can safely delete it as TeX will not use it anyway. (Note: this assumes your personal tree is on a local file system: users with remotely-mounted home folders may still need to hash.)

Regarding MiKTeX, have a look at the section "Installing sty or cls files" in the answer to the question How can I manually install a package on MikTex (Windows) ^[1].

You can then verify what file will be used with:

kpsewhich filename.sty

This will show the path to the file picked up by the TeX implementation.

All of the other answers cover things quite well, but I thought a slightly different version might be helpful.

You have to put it in the 'right' place and second you need to update the database TeX uses to find files. The place to put the file depends on your operating system. Assuming you have a standard installation, this will probably be:

• Windows 10 (and miktex)
  C:\Users\<user name>\Appdata\Local\MikTex\<number>\tex\latex\local\
• Windows Vista/7 C:\Users\<user name>\texmf\tex\latex\local\
• Windows XP C:\Documents and Settings\<user name>\texmf\tex\latex\local\
• Linux ~/texmf/tex/latex/local/
• Mac OS X /Users/<user name>/Library/texmf/tex/latex/local/

Note: the local/ folder might not exist; in this case, go ahead and create it.

A few of notes on that. First, on Windows the 'Users' part of the location is language dependent. Second, I've represented your home/user folder as '<user name>': this will obviously be dependent on your system. The folder I've indicated may well not exist: you may just have the texmf part, bits within that or nothing at all. The file location is important, and although you could just put your file inside texmf/tex/latex is is usual to keep things organised by package. The local folder is reserved for stuff installed on individual machines.

As already mentioned by Arthur, .sty or .cls files must be in some subdirectory of tex\latex which can be in any directory of any drive.

See the following screenshot, it shows how the path to my packages and classes is.

I will explain two cases, registering to MikTeX and registering to TeX Live.

Registering to MikTeX

Open Settings (Admin), select Roots tab, and add my path E:\A\My LaTeX\MyLibrary.

Note that the trailing \tex\latex\misc must not be included !

Afterwards, select General tab and press Refresh FNDB button.

If you cant find MiKTeX Options (Admin) you may have a newer installation that instead uses MiKTex Console. For this version, simply go to the Packages tab and click the database icon.

Registering to TeX Live

See How to register my own packages or classes in a separate drive to TeX Live installation? ^[1]

The .sty files need to be put it some subdirectory of tex/latex in a texmf tree. Those trees have a rather fixed structure, but their location may vary according to your OS and TeX distribution. If you use Unix (including Mac OS) and TeX Live, there's by default a hierarchy

/usr/local/texlive/2009/texmf
/usr/local/texlive/2009/texmf-dist
/usr/local/texlive/texmf-local

as well as other directories. You want to put it in the texmf-local directory, not in the other directories that are managed by the TeX Live installer; when you upgrade TeX Live, your package may be overwritten. The texmf-local hierarchy, as the names hints, is for local packages managed by you; it won't be touched by updates (which is also why it's directly under the texlive directory, and not texlive/2009, for instance).

You may also choose to create a directory called texmf in your home directory, and the package will be visible only to you, not the other users on your computer.

On Windows with MiKTeX, I believe the directory for local installs is C:\Localtexmf by default.

In any case and whatever your distribution, you have to place the .sty file inside .../texmf-local/tex/latex and not directly in the root directory; it's just won't work otherwise.

Then, if you've installed the package in your home directory, you have nothing to do. If you've put in texmf-local, on the other hand, you need to run the utility mktexlsr (or texhash, it's an alias).

On Linux, you can pick any directory you like, and then add a line like

export TEXINPUTS=".:~/latex:"

to your .bashrc file (you can add multiple directories, separated by colons. I think you do need the '.' in the beginning). There may be something similar in Windows. I find this easier, since it separates "my" stuff from the texmf tree. Probably not great if you're writing a big package, but handy for some small things you just want to share between all your projects.

Using texlive on Ubuntu, the *.sty files are located at

/usr/share/texmf-texlive/tex/latex/

or

/usr/share/texmf-dist/tex/latex/

Adding a new style foo.sty usually means

1. create folder foo

   sudo mkdir /usr/share/texmf-texlive/tex/latex/foo

2. copy the file foo.sty there

   sudo cp foo.sty /usr/share/texmf-texlive/tex/latex/foo/

3. run texhash

   sudo texhash

Works for me with texlive 2009-15 on Xubuntu 12.04LTS

The other answers provided to this question all suggest to put your .sty files in the folder /texmf/ in your user profile folder (although there doesn't seem to be any agreement on what exactly you need to do after that).

This will, however, not work if you (like me) are using a portable LaTeX program, because then there won't be any such /texmf/ folder on your computer.

An alternative solution is therefore to put your .sty files (e.g. foo.sty) in any folder you'd like (e.g. C:/foo/), and then simply specify the path where you keep your .sty file when you load it in the preamble:

\usepackage{C:/foo/foo}

To avoid errors or warning messages, be sure to give the path also in your .sty file:

\ProvidesPackage{C:/foo/foo}

To maintain full portability of your LaTeX program, you can put your .sty files in a LaTeX subfolder, e.g. C:/texlive/foo/.

This way you can keep all your .sty files in one single place on your computer, and include the same path with \usepackage in all your .tex documents.

I've just found this in the MiKTeX documentation ^[1]. Just FYI:

6.2. Which is the best directory to keep .sty files where MiKTeX can find them? MiKTeX Package Manager takes care of the proper installation of well known LaTeX packages.

If (for some reason) you have to install a package by hand, then you should copy the input files into the directory tex\latex\misc relative to a registered root directory. For example:

Create the root directory C:\Local TeX Files.

Create the sub-directory tex\latex\misc and copy your input files (*.sty, *.cls) here.

Register the root directory (C:\Local TeX Files) in MiKTeX Options.

For more information, read the section Integrating Local Additions in the MiKTeX manual.

For me, using MAC OS X with standard MacTeX, the above answer were only partially helpful. I couldn't find the directory ~/Library/texmf.

I did the following. Assume that you want to add mystyle.sty to your TeX. Create the directory ~/Library/texmf/tex/latex and place mystyle.sty in it. Afterwards invoke texhash ~/Library/texmf/.

This worked for me.

To avoid tex distribution or operating system specific requirements on where to put stuff and what to run to get it to be found I use a GNUmakefile rule of the following form:

%.sty : ../%.sty
  cp $^ $@

Each subdirectory for which I build latex code references this rule, resulting in the .sty file to be copied locally to that directory before pdflatex is invoked (using suitable dependency rules). See for example, notes/make.rules and notes/phy452/GNUmakefile in my physicsplay github repo ^[1].

This works well for me on Linux (ubuntu + TexLive) and Windows (cygwin + MikTex).

I am using a quick fix to this issue since I work mainly on a USB stick and don't want to touch to the tex folders the computers I am using.

Indeed, you can specify backward directories to \usepackage, then you don't have to know the exact full path but only the relative path from your .tex to your .sty file.

An example: given a portable hard disk D, I have in D:/latex/files/ my .tex files and in D:/latex/packages/ some packages I want to use, then you can write \usepackage{../packages/mypack.sty}

Not sure if this works on windows however

For Windows (10) users on Miktex (Console 4.2):

1. Open Miktex Console (4.2) > Settings > Directories and observe the informational links FAQ: Which is the best directory to keep my .sty files? ^[1] and KB: TEXMF root directories ^[2]. We'll follow the advice in those links.

   

2. Anywhere you like: create a TEXMF root directory, with a structure suitable for your custom latex packages (.sty files). Add your package. E.g. C:\Users\John\Documents\Sda\Code\WritingReadingTech\Markup\Latex\Libraries\texmf\tex\latex\MyPackage\MyPackage.sty

3. Miktex Console (4.2) > Settings > Directories > [+]. Select the TEXMF root directory. E.g. C:\Users\John\Documents\Sda\Code\WritingReadingTech\Markup\Latex\Libraries\texmf. Click [Select Folder]. Observe this folder as in the picture above with "Purposes: Generic", "Attributes: User"

4. In your latex (.tex) source files, existing anywhere on your computer, add a reference to your custom package with \usepackage{MyPackage}. Build/compile your latex source file. Everything should be as desired.

I found it unnecessary to run Miktex Console (4.2) > Tasks > Refresh File Name Database.

This worked for me (Makefile):

KEY := myrepo
TEX := $(shell kpsewhich -var-value=TEXMFHOME)
PWD := $(shell pwd)

link:
  ln -s "$(PWD)" "$(TEX)/$(KEY)"
.PHONY: link

Then:

make link