Since some people are having trouble doing this I created a guide as simple and descriptive as possible, this guide won't work for the following games:
Left 4 Dead 2
Counter Strike: Global Offensive
The model converting process should work for the above, but the maps won't open in the SFM unless you decompile them and recompile with an older version of VBSP. I won't explain how to do that but there's nothing stopping you from doing it.
As for the remaining games and mods, I made a pdf with all the instructions, you're also going to need a batch file and GCFScape.
The Guide: https://dl.dropbox.com/u/44918480/SF...MakerGuide.pdf
The Batch: https://dl.dropbox.com/u/44918480/SFM/fix_models.zip (you'll need to extract it before using, I only zipped it because some browsers tend to open batch files as text instead of downloading, taken from Smashman's thread, this is the same file used to convert L4D models so they could be used in GMod back when there was no official support)