Step 1 - Exclude the .less and .js files that you will not be using.

Step 2 - Edit or override the variable.less file.

Done

It's not quite a guide, but you can customize what to include, and make tweaks to fonts, spacing and the like, directly on the bootstrap site:

http://twitter.github.io/bootstrap/2.3.2/customize.html

It's fairly self-explanatory, and the resulting file will contain only the code you actually use, making for smaller file sizes.