The Lego Digital Designer ^[1] is ideal for this job. Avalible for Mac and PC it allows you to build models from scatch, order them, and get them delivered. Or, if you've got the bits already it can make instructions for you. It has a range of technic items (see image below).

Importing a model will also enable you to view the part number and details which could be very useful for finding out exact pieces/parts.

I tried LDD at first, but frankly, the order in which it adds the parts is often quite nonsensical:

I also tried a bunch of other tools, some of which are mentioned elsewhere on this page, with mixed results.

So what I ended up using, with great success, is LIC (LEGO Instruction Creator) ^[1]. It's beta, somewhat buggy, and for all I can see it was written by just one AFOL in his spare time, and the development has stopped. But it's very easy to use, quite versatile, and I am more than happy with the results. Here's what it looks like:

(Scroll down for sample images of the final instructions for two models.)

Basically, it's a WYSIWYG editor that takes an LDraw ^[2] model as input. And a surprisingly powerful one at that. Some of the things you can do with it are:

• switch between vertical and horizontal layout;
• add, delete, or merge steps;
• move individual parts between the steps;
• move everything around the page freely;
• rotate individual parts or the entire model, automatically adding rotation icons if you wish;
• "displace parts with arrows" (just like in TLG's instructions, where the part has not been connected to the model yet, but an arrow shows where it's supposed to go and which way around) — this can be done in any direction, up or down, left and right, top and bottom, and for several parts, too;
• make a template for your instructions (things like fonts, font sizes, background color, and so on and so forth), so instructions for different models have the same, your very personal, look and feel;
• add text labels such as part counts, your name, really anything.

LIC will also automatically create a parts list as the last page of the instructions. Allegedly it can also export to PDF, and integrates with POV-Ray for higher-quality images, but sadly neither of these options works for me (as I said, there are some bugs). Might work on a different machine, though, or a more recent OS. I don't know.

So anyway, my complete toolchain looks as follows:

1. MLCAD/LDraw ^[3] for modelling. It has a steep learning curve, but once you have figured it out, it's a breeze.
2. Then I feed the LDraw model into LIC, which generates a draft of the instructions and allows me to fine-tune them interactively.
3. Then I save the instructions as images and do some final retouching in GIMP ^[4] - moving stuff around a couple pixels, adding photos, stitching pages together, etc.

The final result can be seen on my Brickshelf ^[5]. Here are just two sample pages:

All tools I mentioned are free and available for download. Follow the links.

See AlternativeTo's list ^[1].

• LEGO Digital Designer (official; most popular)
• LDraw (2nd most popular)
• Konstruktor
• MLCAD

You can do all this in Google Sketchup with SketchyPhysics simulation, of course! And the pieces are easier to fit in, and GS is generally more flexible and easier to use, unlike LDD. The disadvantage is, you don't get LDD's Building Instruction Generation capabilities.

(You can download bricks for Sketchup from the online gallery. For example, LEGO Mindstorms NXT parts ^[2].)

LPub

LPub ^[1] is a program that allows you to produces step by step instructions for models. It's an alternative to the built-in Building Guide Mode featured in Lego Digital Designer which produces a set of instructions automatically (although not necessarily in the most logical way).

LDD model instructions can be created in LPub provided that the parts in the LDD model are also available in the LDraw library (cf. this related question ^[2]). An .lxf file can be opened in LDD and exported as an LDraw file (.ldr). The LDraw file you exported using LDD can be opened in LPub.

You should look at LPub4 ^[1]. This is an OpenSource (GPL) tool that can create high quality instructions from LDRAW compatible files. It runs on Windows and OS X and with my patches ^[2] on Linux as well.

Note that the LDD license states "Any commercial use of the software is strictly prohibited" ^[3] so keep that in mind if you are intending to do anything commercial with your instructions (such as selling them). GPL software such as LPub can be used commercially, though you do need to make sure to understand the license obligations it carries

BrickLink Studio ^[1] is a design tool to build models (like LEGO Digital Designer) and includes the ability to create instructions as PDF or images. Therefore the models are divided into steps via the modeller or in the step builder (part of the instruction maker).

• Steps can be reordered
• Parts can be moved between steps
• Parts can be grouped into submodels with their own steps (these can be converted to so-called "callouts" so that they don't appear in the normal steps ordering but in a separate box within another step)
• Instruction pages are generated automatically from steps
• View/angle/zoom of model can be changed on a per page basis (kept until the next view change)
• Elements on pages can be moved
• Brick insertions can be visualized by displacing the brick with an arrow indicator in all directions
• Manual insertion of elements like images, rotaion symbol, free-text fields ...
• For some layout elements presets can be defined and saved into a configuration
• Parts list

If you want to pay some money for professional software, I'm pretty sure that SolidWorks has a complete set of LEGO parts that you can use.

http://www.solidworks.com/

The LDRAW package contains a tool called LPUB.

If you design your LDR files with various "step" instructions, you can have it generate a PDF with one page per step.

Some tools, such as BrickSmith (http://bricksmith.sourceforge.net) present steps in a hierarchical fashion so that you can step through them. Others, such as MLCAD, show them as breaks between the various steps.

Bricklink Studio ^[1]. It generates good instructions like these: ^[2] It's also very straightforward to use: