Lay the sheet of plywood on a flat surface, like a workbench or across two sawhorses.
Measure four feet from one end of the plywood's 8-foot length to find the center. Mark it with a dot. This determines the center of the scallop at the lowest part on a fence line.
Lay the measuring tape across the plywood's 4-foot width on the center dot. Mark an "X" at a distance for the depth of each scallop. For this example, mark the "X" at the 10-inch mark on the measuring tape, measured from the side of the plywood.
Lay an 8-foot length of quarter-round trim on the plywood, aligning its center with the "X" mark. Nail the trim on the "X" mark with a hammer so it is held in place but the nail does not go through the plywood.
Bend the left side of the trim to the edge of the plywood on the left side of the board. This will be the left corner of the board nearest the "X" mark. Tack the trim to the corner with a nail and hammer. Repeat this process from the right side of the trim and plywood.
Trace a line on the plywood with a marker to follow the curve of the trim.
Place the claw end of a hammer under each of the three nails. Remove them with the trim.
Cut along the curved outline with a reciprocating saw to create a scallop template.
Hold the template up to a fence section, with the right and left sides touching the top fence line between two corner posts. Trace the dip in the pattern onto the fence board.
Cut along the marking with a reciprocating saw to make a scallop.
Repeat the process of holding the template to a fence section, marking the curve and cutting to form a scallop, until the entire fence is completed.