Here is what you'll need:
- Cardboard letters (I got mine from Hobbycraft but they're widely available online)
- Mod Podge (again, Hobbycraft) sparkly and plain
- Something to cut up for the pictures. Comics are great but I had a couple of these rubbish Hello Kitty books to butcher for the girls' letters
Cover your letter with your choice of background paper. It doesn't have to be plain - spots or other wrapping paper could also work well. Cutting squares around 3cm x 3cm is an easy way to patchwork it, rather than trying to wrap it. Let it dry (it doesn't take long).
Then you need to cover your letter with your pictures of choice. Here is one partially completed.
... and here are two I made earlier. Give it all a final coat of Mod Podge which dries like a varnish to seal and protect everything. My boys have these on their doors stuck on with those sticky Velcro dots but they can also stand on their own on a shelf.