TAG SETUP INSTRUCTIONS:
put any custom tags that you want to use in the "logos" folder in your half-life directory. then you'll be able to select them from the setup menu and use them in the game.

for technical info on making logos try: 
http://www.hardwarepub.com/articles/logos/logos.shtml
http://connect.to/deviln
http://www.karljones.com/halflife/decals.asp