-Something to decoupage on: Old shoeboxes can be a great place to start (and are free so there's nothing lost if you screw it up) but pretty much anything can be decoupaged as I'm sure you've seen around here. (Wooden items like tables or cigar boxes, plastics like suitcases, metals like the ever popular Altoid tin, glass vases and such - if it holds still it can be decoupaged.) For this I used a small chocolates box.
-Something to decoupage onto your object: This can be pictures from magazines, pretty paper napkins, stamps, some fabrics, comic books, old sewing patterns, etc. It's generally best to stick with something in the fiber family (papers and fabrics) and be fairly thin because you want your glue to get a really good hold. For this I used images of print fabrics from catalogs and magazines.
-Decoupage medium: You can use Mod Podge (available in matte or shiny finishes), other commercial products or just plain white school glue watered down (equal parts glue and water), which is what I like to use.
-Brush for the medium
-Scissors: To trim up your images... I recommend using scissors that you don't mind getting a bit messy since you're dealing with glue.
-OPTIONAL Brush for paint & paint: I like to paint my object before I start just in case I don't completely cover the surface with my images or in case they are transparent. I used white acrylic craft paint.
1. Prep your surface. In my case I peeled the label off my chocolate box since it wasn't glued down very well and I didn't want the logo showing through and then I gave it a quick coat of paint.
2. Figure out your design. If you are doing a complete collage, select the images you want to stand out the most and set them aside. You'll want to save them for the end so they don't get buried under other images.
3. Start somewhere... slop down a whole bunch of glue on your surface or to the back of the image itself. Apply your image and slop down even more glue on top. The idea is to get your image completely sealed in glue, top and bottom, so that there are no air bubbles or lumps.
You may notice with some images (specifically those cut from magazines) that you can see the printing from the opposite side when they are wet. In my experience, if you couldn't see it before the paper was wet, you won't see it once it dries, so don't freak out until you see how it dries. Keep moving until your item is covered in images to your liking.
4. Let dry.
5. Trim up the edges of your object if applicable.
6. Cover the object in glue medium again and allow to dry.
7. Check your object for any loose images... places where a bubble may have formed or a corner that is sticking up. I glue these down with full strength white glue (which dries clear.) OPTIONAL: You can lightly sand the object with a very fine sand paper to remove any extra lumps. (I never sand my stuff though.)
8. Cover in a 3rd coat of glue and allow to dry. I usually do a 4th and 5th coat as well. In between coats for the patchwork box top, I decoupaged the bottom of the box with pink tissue paper and made a felt lining for it.
The lid for my box was rather loose to begin with but in case of a tight fit between box lid and bottom, decoupaging the bottom (or inside of the lid) wouldn't work because the box would no longer fit together. It's important to keep that in mind when decoupaging boxes.
9. Seal if desired. Mod podge is made to act as a sealant and in my experience school glue is fairly durable as well. However, if your creation will get a lot of wear, you may also wish to spray it with a clear art sealant.
10. Enjoy your finished creation!!!
Decoupage is not an exact art - there can be a lot of trial and error depending on your medium, the object you're decoupaging and the images you are using. For this reason, I wouldn't use any antique or otherwise irreplaceable items for your first project. Overall though, it is a very versatile and forgiving craft.