You can simply paint the desk if you have made it from inexpensive materials or you can do a little better job on the Finishing if you have used hardwood. Well and underside of desk Paint the Lower Shelf if it is not hardwood Sand and round corners. Prim with thinned shellac Sand with fine paper Paint with oil based finish Sand with fine paper Paint with a second coat of finish Plywood Edges Plywood edges take some effort to seal. The end result should be uniform in texture and not have holes one place and excess filler another. First fill all visible holes with glue and wood splinters.
Toothpicks work well.
Let dry and sand with course paper over a wooden block. Let this dry thoroughly and sand again. Seal with a thinned coat of shellac. Sand again and refill. Sand again, seal again, and sand with fine sand paper.