First I made my own decoupage glue. There is a brand called Modge Podge, supposedly it works great. However, the price is not so great at all! I opted for a homemade version I found online. Ingredients: equal parts water and Elmer's glue. How simple and inexpensive. I mixed up my glue in a bowl but you could easily use a plastic cup (easy disposal). I was surprised at how little I actually used.
I was making a picture frame and wanted to decoupage squares of scrapbook paper onto it. I dipped the squares of paper into a bowl of water and placed it on the frame. I bent and folded the paper so that it would go all the way around the edges. I then brushed a layer of glue over the paper. I used a sponge brush, which is a black sponge on a rod (they are super cheap at craft stores). I did square by square the same, adding a layer of glue. I waited for the layer to dry and then continued brushing on glue one layer at a time. I probably did upwards of five layers of glue. I then added a few embellishments and decoupages over those as well.
Decoupaging was SO easy, I was surprised actually that I hadn't tried it earlier. This process could easily be used on so many different surfaces. Since I made just a frame I let the decoupage glue be my sealer. However, if you were going a whole piece of furniture you would want to add a layer of sealer on top to protect the furniture.