It's really easy - I saw it done ages and ages ago on the Christopher Lowell show, took some notes and did it myself.
Galvanized flashing (if you can find it cut to size at a non home depot store that sells metal you'll be golden!)
tin snips or a heavy duty boxcutter (and something to make a stable straight edge to score the flashing against and lots of cardboard for underneath)
heavy work gloves (flashing is sharp)
linoleum adhesive and a putty knife to apply it with
upholstery tacks, screws or something else to hold down the sides with plus a hammer or screwdriver.
[I would highly recommend wrapping the flashing around a thick piece of plywood and then attaching that to the top of your desk because it will limit the sharp edges you may wind up with otherwise]
cut the flashing to size (you'll want to make it bigger than the top of the desk so that you can fold down the edges over the lip of the desk - or to wrap the flashing around the plywood)
Put on gloves before cutting!
To cut with box cutters you'll want to score the flashing numerous times to that you'll be able to bend the metal along the cut and it will snap along that scoring - this takes a lot more scoring than you'd think!
To cut with tin snips you'll want to go really slowly so that you don't get a ragged edge.
Apply linoleum adhesive (this stuff is really really noxious - give yourself plenty of ventilation) liberally to your desk top or plywood according to directions on the can and wait a little while for the adhesive to get nice and tacky - then position the flashing over the top and press it down flat onto the top but don't bend it yet - you'll have to leave it overnight with heavy books etc stacked on top to get a nice flat surface.
After the adhesive has set you can bend down or wrap the edges (you'll have to do some gift wrap folds or use tin snips to make cuts at the corners) - use the upholstery tacks, brads or screws to hold edges in place.