Never made any custom content before? Start with the easiest, most basic stuff (which you can make with all free programs) - Sims 2: Walls, Floors, and Ground Cover, Sims 3: Custom Patterns
Just installed the Sims 3 patch or about to? Read this article for helpful advice about patching and keeping your custom content working!
If you would like to make defaults of my eyes and share them, please do! All I ask is that you give me credit. ^^
I'd also appreciate it if you didn't edit/recolor my textures without my permission.