Go to PlanetQuake.com and get PakScape.
You must decide wich models are you going to use. Razor´s a good choice for human-like guys. If you patched Q3A up to 1.17 and above, you can use the developer´s models (Carmack, Antknow, etc)

Search into the main pk3, and look for models/players/"the one you want"/ and export the skins to the hard drive. They´re TGA at 24 bits. Edit them as you wish.
Now, to add your friends to Q3A, you have to dump all the other files located in that same dir, the models, the skinning parameters, and the animations stuff, etc.

Make a new pak with pakscape and create the folders models/players/"what ever you want" and dump all the models, the skins you made, and the animation cfg, but NOT the .skin files.
You must edit these with Notepad, there´s a couple of wrong paths and filenames in there, you must adapt them to the name of the skins.
For example :

you have for the model Razor these skins :

h_john.tga (the head)
john.tga (the rest of the body)

h_mike.tga
mike.tga

h_idiot.tga
idiot.tga

You must have these files (grab the originals, duplicate as needed, and change the names acordingly)

head_john.skin
upper_john.skin
lower_john.skin

etc, etc...

REMEMBER : Some of these sets of .skin must be named :
head_default.skin
upper_default.skin
lower_default.skin

Otherwise, the game wont be able to tell wich skin is the default one.

You must edit each .skin
look inside, you´ll find strings like that :

inside head_john.skin :

"h_goggles,models/players/razor/patriot_gogs.tga
h_head,models/players/razor/h_patriot.tga
tag_head,"

there´s some mistakes there, change the folder and file names until it matches the truth.

It´s a little complex, i´ll suggest you to look at some user made model paks, so you can see how it looks like. If you want to add shaders, that´s a different story. You must add a script folder and throw inside the shader definition. Check for the PQ tutorials for that kind of stuff...

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FTF Software