Decoupage is actually quite simple! If you can cut and paste, you already know most of the techniques involved. Basically, you cut out pictures; you glue the pictures onto an object; and then cover the object and pictures with a few coats of glue/decoupage medium to protect it.
Of course, you can buy supplies and other items that are made specifically for decoupage. One of the nice things about this craft, however, is that you can also use supplies that you probably have laying around the house right now. Just look at some of these pictures of finished decoupage projects.
Now, start gathering those supplies....
