In the mapping FAQ eDan Co. said:
Q: How do I make my own textures?
You'll need to download and install program called Wally.
1. Make a .bmp of what you want you texture to be.
(The texture size needs to be multiplications of 16. Examples: 16x16 pixels, 32x16 pixels, 96x96 pixels, 80x64 pixels... and so on...)
2. Open Wally and go to File->New.
3. A window titled 'Create New Texture' will open. You can set only one thing in that window- 'Type'. Set the 'Type' to Half-Life Package [wad3] (.wad)
4. Drag and drop the .bmp you made on to your New Texture in wally. You can add as many textures as you want to your .wad file.
5. Save your file in you Counter-Strike directory.
6. Load the texture in Hammer. (Tools-> Options-> Textures-> ADD WAD.)