Sand the desk to create a smooth surface then paint the surface with a base color, if desired.
Lay out the design of the collage on a flat surface to get an idea of where to place each piece of the collage. The pieces should overlap slightly so there are no gaps in the collage. Leave the design in place and add to the desk one piece at a time.
Brush a medium-thick coat of decoupage medium onto the desk.
Lay the paper or picture for the collage in place on top of the decoupage medium layer.
Smooth out bubbles by hand, pushing from the center toward the outer edges. A rubber brayer, or roller, works well for large collage pieces.
Continue adding the other elements of the collage until the desk is covered.
Brush a light coat of decoupage medium over the entire collage and allow to completely dry.
Sand the surface with wet, fine-grit sandpaper, and then apply at least one more coat of decoupage medium. Add as many additional coats of decoupage medium as you desire, sanding between each coat.
Rub the desk with steel wool to finish the final coat of decoupage medium.