If at all possible, remove the mailbox from the post. If you can't you'll have to tape paper around it to protect the post.
Smooth out an areas that need it with sandpaper. You can also remove any rust at this point as well.
Clean the mailbox with mild soap and water. It helps to use warmer water and a scrub brush. Rinse with water and let dry.
If there is any flaking paint or rust, sand smooth with sandpaper.
If you can, spray with a white primer as a base. Hold the can 6-8 inches from the surface and use a sweeping motion. Apply several light coats, allowing to dry between coats.
Apply your selected color to the mailbox. Use an outdoor spray paint . Add a total of three coats and let stand 24 hours. Remember to keep the can 6-8 inches, and the coats should be light. Use a sweeping motion.
Make sure the mailbox is completely dry before you remove the painter's tape. Replace the mailbox if you removed it from the post.
To decorate the inside of the mailbox, add a layer of Mod Podge to the inside. Gently smooth down the scrapbook paper and let dry. Go over the inside of the mailbox again with Mod Podge and let dry completely.
Notes
Step 8: I decided against doing the entire inside of the mailbox. I didn't want to go overboard with the lobsters. ;-)