the easiest way to get into making custom props for hl2 is to go practice making some for hl1.
the actual .smd file that you need to export from your modelling program (milkshape ftw) is the same for either engine, so a project like customizing some of your old tfc or cs flags, weapons, resupply bags etc, would allow you to learn more about model making, and less about the extra confusion caused by

• messing about with strange compilers and text files to make textures (hl1 uses simple bmp's)
• the need for seperate texture directories (the simple bmp textures can be embedded into hl1 models)
• what to do about accompanying dxlevels or lods to bloat the directories (good old hl1 models are stand-alone)
• all sorts of other weird s**t

fair enough, much of the 'weird s**t' is what makes hl2 possible, but producing models for hl1 is much less hassle, and still gives you a solid grounding for stepping up to hl2. (and a collection of models to port " SRC="images/smiles/icon_wink.gif">)