Turn your vanity upside down on a hard, flat and even surface so the bottom of the vanity is facing up. Place a straight top hardware appliance on each corner of the vanity.
Measure each square metal hardware appliance so that it sits in half an inch from each corner edge. These appliances are screwed into each corner of your vanity and the screw legs are then screwed into the appliances. Mark each screw hole in each hardware appliance with your pencil. Remove the hardware appliances from the bottom of the vanity.
Drill pilot holes for all of your screws by drilling each a screw into each of your marked pencil holes about 1/4 inch. This will help prevent the wood in the vanity from cracking or splitting when you drill the screws all the way in.
Put the hardware appliances back in place so that the screw holes of the appliances match up with the screw holes you drilled. Drill each screw down into its screw hole until it is just tightened. Use the screwdriver to tighten each screw down until it will no longer turn.
Lay each of your screw-on legs on a piece of newspaper. Use spray paint to paint the legs the color you want them or brush stain onto each leg to make each leg the same color as your vanity. Repeat your paint or stain application until you have your desired color. Let the legs dry completely.
Screw each leg into the center threaded area of each of the hardware appliances. Turn each one firmly until you can turn them no more. Turn your vanity over and put it back in place in your room on its new legs.