check the filebase for open source bots, or some hpb bot template. There is no step by step guide, no document about the interface to HL, but the template or the open source bots will help you a lot. Most stuff can be found there, but you might still encounter stuff where you have to find out the mechanisms yourself.
the template :
http://filebase.bots-united.com/pafi...ion=file&id=92
metamod / sdk folder :
http://filebase.bots-united.com/pafi...=category&id=5
open source bots :
http://hpb_bot.bots-united.org ( the father of the bots )
http://joebot.bots-united.org ( well, my bot. 2 years working on it left marks, so the code is partly unstructured, because I didnt rewrite it from scratch, but continued to extend botman's template #1 - currently I'm workin on a new bot, but that's not yet open source )
http://realbot.bots-united.org ( stefan just went open source with his bot ... check the wiki and the realbot forums for details )
http://racc.bots-united.org ( good ol' ( well, in hl bot programming he isnt that old ) PMB's site with tons of metamod plugins ... as I already stated elsewhere, nobody knows where he got his time from ... looks like he goes all the time with almost light speed so time dilatation has an effect
)
and the ones I forgot, or dunno the current url of
Learning by doing, nothing more, nothing less - and don't despair when you are searching bugs for days or weeks, but since you say you are an experienced gfx programmer, I assume you are used to searching bugs for weeks