1. Open minecraft.jar.
2. Copy "terrain.png"; save copy to a separate location.
3. Open the terrain.png copy in an image manipulation program (GIMP, Photoshop, etc.)
4. Edit the block textures.
5. Save and zip the file; place in the minecraft texturepacks folder.
6. Run Minecraft; select your texturepack.
7. ?????
8. PROFIT.

Sorry. Something compelled me to include steps 7 and 8. I don't know.

Watch the video here by randommacfive. Its out of date, but the basics are the same,

Go on youtube, type "How to make a 1.4.5 Minecraft Texture Pack," And there are several, just click the one take looks... c: On utube, just judge