The first step of these free woodworking plans is to cut the plywood for the shelf.
To cut the plywood, you may use a table saw or a circular saw with a clamped straight-edge (as shown in the picture on this page).
The dimensions to which you should cut the plywood will depend on the final size of shelf required for your installation. However, keep in mind that we'll be wrapping the plywood with 1x2 in an upcoming step, so allow for 3/4" on the front and each side of the shelf when determining the size to cut your plywood.
In my case, the shelf brackets were 12" wide, so I wanted a 16" wide shelf that was 48" long. As such, I cut my plywood to 15-1/4" wide x 46-1/2" long.
NOTE: The reason I recommend cutting the shelf plywood before installing the shelf brackets is that in some installations, holding the plywood in place on the wall is a good visual cue when determining the locations for the shelf brackets in the next step.