Interior design is great fun. It can be expensive and an awful lot of effort to do in real life, however, so I think I'll stick to doing it virtually for now. The latest Sims 4 pack should help with that, because it adds interior decorator as a new in-game career. Out now, the Dream Home Decorator expansion will have you picking out cool furniture for other sims, and prettying up their homes to satisfy their requests.
I'm honestly surprised it took this long for The Sims to get this sort of career. One of the best parts about the game is designing your own homes (or trying to recreate interesting ones from other games).
As a sim interior decorator, you'll need to take into account your client's budget and preferences to tailor your designs to them. You can have a little fun with it, of course, but it's still a job. If you want more customers you'll need to make sure you're actually giving them what they want - even if it means using really gaudy decorations.
Sometimes you'll only be tasked to change one room, others you'll get to redesign a whole house. At the end of it all there's a big reveal too, like some reality TV show, where you'll unveil your work to your clients (and hope that those tears are happy tears). You'll get to see a before and after comparison too, which is cool. It's nice to look back and see how much you've improved things.
Fair warning: it's probably best to do exactly as your clients tell you, or you could end up with some rather ominous feedback: