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!
Obviously I don't make objects but this applies to my houses and CAS screens, please DO NOT upload them to the exchange or to other sites and say they are yours. I don't want to see my houses credited to someone else or there will be hell to pay.