Lose the lids. (No! Keep them! Not for this project, but surely there's something to be done with them!)
Use the sheet metal screws to attach the four L brackets around the openings of each trash can. Imagine the round top of the can as a clock, and affix the screws at 12 o'clock, 3 o'clock, 6 o'clock, and 9 o'clock on the outside of the can. When you're done, it should look like there are four little diving boards sticking out of the trashcan.
Stuck up
Glue the galvanized steel to the plywood with contact cement. You might want to wear latex gloves during the gluing process - the stuff is powerful, and while it won't really hurt your skin, it's a pain in the neck to wash off! This will be your desktop.
Take your friend out for a quick coffee while the cement fully dries.