It depends on what type of mod you're trying to do and which program you're using. I find that I get a lot of problems solved by using Google search and looking for tutorials on what I want to do.

For example, if I want to make a room a different color and I'm using Photoshop, I can type "How to change colors in Photoshop" in Google. I did this because it would take a long time to make specific guides on mods.. there's so much to do!