What you'll need.
Some sort of boards to create a solid backing for the decoupage application.
Screws to attach the board
Primer (we used Kilz as it's inexpensive and does a great job)
The paper, whatever you choose to use.
White glue (we bought lapage in the four litre bottle for $15)
Foam brushes (get lots of them, they're cheap and they'll break if you're doing the 200+ sheets like we did)
Trim (metal, wood, whatever you want)
Adhesive (to attach the trim: we used liquid nails)
Step 1: Strengthen and Prep the Surface
We pre-drilled the holes about every 6-8 inches. You can't just screw it in as it'll crush the board. Take the time and pre-drill.
We screwed the boards in and we're relieved to find that they were cut perfectly and fit as they should.
We mounted all the boards and than vacuumed and wiped it down in preparation for the primer.
We primed it with Kilz primer. The only thing we should have done is taped the joins with masking tape first and then painted. Then the wall would have been seamless. The wet paper likes to get into the smallest crack and bubble.
Step 2: Get Your Paper Ready
Whatever you've choose to use get it ready in advance. It'll save you headache in the end. Pay attention to things that'll stick out like bar codes. We used sheet music, original compositions, a few scores, jazz charts, photocopied a few pieces that meant something to us (and we didn't want to destroy the original) and tried to represent a range of music genres.
Step 3: Start the Decoupaging
To decoupage mix the white glue (in a container) with some water. We found that 1 part water to 4 parts glue worked the best. Not too watery but not too thick.
Lay out some wax paper to work on. It works nicely to reduce the mess. Replace the wax paper every time it gets to tacky and messy.
Take a piece of paper and using the foam brush paint the back side of the paper with the glue mixture (on the wax paper). Paint a patch roughly the same size as your paper on the wall.
Put the paper on the wall where you've just painted and apply more glue the surface of the paper working from one corner and trying to get rid of as many bubbles as possible. Repeat. Repeat. Repeat. Etc.
Note: once the glue dries the paper will tighten up and many of the small air bubbles will disappear.
Keep going. Overlap as much as you want to. We decided to keep everything level but you could certainly turn the paper in different orientations as well.
Step 4: Seal It.
Step 5: Add Trim and Smile.
And that's that. All done.