Hi there, and welcome to my skin how-to. This is just for using the damn things, not making em. I'll make a guide for that later. First, lets start with the basics.
You'll need to download a crack to be able to change the name of the main folder. I feel really bad about using the crack, but if id won't let us do it, we have to MAKE them let us do it. It can be found here. It is for the 1.08 test only.
Then, rename the "demoq3" folder to "baseq3". This takes it out of restricted demo mode and into real Quake III mode.
Now, you have to create 3 folders in the newly renamed "baseq3" folder. They are: Models > Players > Visor (and/or Sarge for some skins). This is the directory where you will need to put your skin files.
Then, put the (skinname).tga, (skinname)_lower.skin, (skinname)_upper.skin, and (skinname)_head.skin files in the applicable Sarge or Visor model folders.
Now, fire up Quake III. Create a game server (or join one) and go into the console (best to do when you get killed).
Type in "model visor(or sarge)/(skinname). The skin's name is the name of the tga file in the models folder.
Eg. "model visor/khaz" DO NOT PUT "model visor/khaz.tga, models visor/khaz," or other combinations. These never work. Type it in exactly as I've shown you.
Want to try it out? Download my skin here and get the Crack here.