• Creating New Textures Using Wally
• Using the Rivet tool
• Using the Bullet Hole tool
• Using the Patterned Paint tool #1
• Using the Patterned Paint tool #2
• Using the Decal tool
• Creating a custom decal texture (advanced)

• Working with WAD files in Wally
• Creating Half-Life Color Decals (Counter-Strike, etc.)
• Using the WAD Merge Tool
• How to send WAD files to clients (aka, working with RES files)

Some excellent external tutorials:

• Texture Central