Measure your microwave to find the dimensions your shelf needs to be to accommodate it. Add an extra 3 inches to the depth and length of the microwave to get your shelf dimensions.
For example, a 24-inch-long by 15-inch-deep microwave will sit comfortably on a 18- by 27-inch shelf.
Cut out the shelf dimension from a piece of three-quarter-inch plywood using a saw, or alternatively have the lumber yard cut it to size for you when you purchase the plywood.
Paint the plywood in the desired color on both sides and let dry completely before continuing with the installation.
Locate two adjacent wall studs – they will be 16 inches apart – in the position you want to install the shelf and mark a level line between them at the height you want the shelf to be.
Place a heavy-duty shelf bracket on the stud point, with the angle of the bracket even with the wall line you marked. Secure the brackets to the wall and the wall studs with 3-inch screws driven through the screw holes in the brackets.
Place the painted plywood shelf on top of the bracket arms, centered. Secure the shelf to the brackets with half-inch screws driven through the screw holes in the bracket arms into the microwave shelf.