Right im going to offer up some of the experience ive had modding PEDS as it also took me some serious time to get my head around it with a lot of trial & error, and some things ive just had to learn for myself. Yet i keep hearing others struggling with this concept so im willing to tell what ive learned BUT! before you even start STOP!! reading any further
if you haven't already read and understood this tutorial http://www.gtaforums.com/index.php?showtopic=481411&st=0
because if it wasn't for this i wouldn't know jack sh*t! as i was a total GTA IV modding & 3ds MAX noob so if i can do this they you can too!.
there are 2 types of peds the common PED & the story type PED.
The story type ped is basically either a clone of the player model or the ones you see in cutscenes they both have compleatly sepatra meshes just like your player model probely because they need to be of a higher quality as they get seen up close more and need to stand up to scrutiny. Either way some have very basic parts (EG) johnny a single mesh for each body part! & some have multiple parts like Tony because they have the ability to change outfits & each multiple of the same mesh is an different item of clothing, Rigging for these is very simple and is where i belive you should start!. check out the johnny ped pic for my idea of simple.
Like i said its simple its basically the same way you rig your player model and if you have already have done that its a straight cut a paste method because those meshes are already aligned to fit the story ped perfectly. But what you have to watch out for is the ones with the multiple items of clothing be sure to remember that only those meshes that the game calls to load for that scenario will show up so you can get away with only rigging the _000_ range of files as they are the default ones notice how the head & hands are also in that range. but if you do it like that & that ped is called in a cutscene where he should be wearing one of the other types like _001_ they that just wont show up at all or could cause an issue maybe? But like i said if that ped is just following you around as a bodyguard then the default _000_ files will be just fine.Infact you could even rigg differant clothes to the other number ranges for costume changes.
The short version is that most of the time all of the parts to compleate that outfit/costume are in the same number range. see TONY below.
This is where it can get confusing belive me ive been there!
Almost all common ped types are lower in number of meshes & have some parts joined that will make a noob scratch there head wondering what to do about that EG: the feet & lower are now one mesh, the head & hands are attacthed.
For the lower its not an issue you simply have to attatch your imported models feet to the legs before you import that model in to the 3ds max scene or you original had one like that you previously divided for the player rigging so you still have a copy of it like that.
Head & hands being attatched is not an issue really one thing ive never done or never will is rigg the hands what is the point in that! hands are hands & no matter how good you are @ it you will never get it as good as the ones in game at holding guns and stuff.
This is what i do!
Import the ped model you want to change and deleate everything apart from the attatched _000_ head n hands then import your custom head in to the scene and attatch it to the head in the same way as you learned in going gods tut, great! now you have a custom head with identical hands attatched just like the ped has, and those hands should rig perfectly as they will be perfectly allinged to the bones envelope's like the ones yor going to replace. So save that file as your new head n hands template.
The real problem ive found with common peds is they have more than 1 of the same part just like the tony ped some may be differant clothes some will be differant heads!! ,thats all because these peds are randomly genorated as you drive around and to stop everyone looking like a clone they have differant heads . the problem ive found here is as you never know which head or costume the game engine will call to load & say for exsample you want to change to police models to something fixed then you will have to rig every single head and outfit with the same custom stuff you want or it could be just a randomized mess of parts. I would recomend doing this by attatching it to a part then instantly hiding it so it dose not get confusing or messed up after and you know what part your working on.
you may also notice that there are some low poly meshes these can be identified as ending with _med NOT _high! I belive thes get called to load either on low setting or as the ped is further in the draw distance it switched to the low poly model to save resorses ive seen the helmet on the swat model reapeaar as he was further away because i didnt bother with them, rig em to if in doudt.
Now here comes the fun part !
After your all done rigging everything and you have set all your texture ID;S to 1 on each mesh by going editeble mesh , poly CTRL+A to highlight it all red then set ID TO 1.
then done your material stuff and set each of them ID'S also to 1
it should be time to export the model and if all goes well import it with OPEN IV replacing the original PED wdd then ALSO OPENING THE WTD texture file of the ped and with Edit mode on replace every single texture in it with your custom ped textures to the parts where its needed.
time to test:)))))
One thing i will say before you even do a thing if you want to avoid any silly export errors because i also used to get the gemoerty & dev model rubbish error's, the trick is dont mess with any of that as i find most of it was from importing stuff in to the rigging project dirty AKA not clean in other words your custom mesh you imported had exess rubbish attatched to it like previous dev model, skeleton info basically anything , so before you import that part Make sure you hit that hightlited button to to pen the list selecte everything thats not your mesh ( the circle object!)
and hit the deleate key until all thats in it i that single circle icon , then its good to import to any rigginf project without fear.
GOOD LUCK & A PREY THIS HELPS BECAUSE my fingers are now hurting