One of my favorite things to do for 3-D decoupage is buy packages of cards from the dollar store. I look for cards with well defined lines so they are easier to cut. Here are a couple I made this afternoon.
These were made with only three layers. I usually like to use more layers, but I needed these to fit into an envelope for mailing.
If you've never done 3-D decoupage before, using cards is a great way to start.