Measure the dimensions of the sink bowl to be installed. Mark that area on top of the desk using a pencil.
Drill a hole in the center of the sink area using a drill bit wider than the width of your jigsaw blade.
Insert the blade of your jigsaw into the hole just drilled. Cut a line from the hole to the border of your sink area, then turn the blade to cut out the area for the sink.
Sand any rough protrusions caused by the sawing.
Apply a coat of waterproofing sealant to the top of your desk. You also might want to coat all surfaces on the underside of the footwell.
Move the desk into position. Install the sink according to the directions for the make and model.