The process itself is easy! Some people use white glue, but I prefer decoupage medium: it's easy to work with, and give a very nice over-glaze when you cover everything up with that medium when your project is complet. It goes on milky, but dries very clear. Not sure how it'll take wet drink glasses if you're going to use it for a bar tray, but if you want that kind of protection, you could use a pourable resin that will level off your different thickness of papers and provide excellent protection. However, it does make for a heavy finish, so you'd have to make sure your board isn't flimsy, or poorly attached to the sides. You may need to reinforce the bottom. Another option is to do your decoupage, and have an acrylic sheet or glass cut to size and placed over it. It would be easy to remove to clean.

Enjoy the process...just jump right in! ;-D

Thanks, Glitter! That's exactly what I'll do. I'll get some glass. Reinforcing the bottom might make the tray too heavy if I decide to go with the glass. We'll see after all is finished.
What medium (brand) do you prefer? I'd like to have a high gloss look, if possible.

Hm. I can't honestly tell you which brand, but it's the one usually carried in Michael's..white plastic bottle; use those small, cheapie sponge brushes and toss 'em after you're done! Be careful if using glass, as it can be quite heavy for the tray....glass places can usually cut acrylic also, which can be much lighter.

Anyway....enjoy the process!! And yes...the high-gloss gives a nice finish...several coats over your masterpiece, making sure to dry between coats would be great (may not even need the glass, who knows?). Just don't panic when it goes on white...it'll dry clear.

Have fun!