Snap a chalk line from corner to corner on one of the pine boards so that the line divides the board into two equal triangular halves. Position the two saw horses so they are aligned and 3 feet apart. Clamp the board flat onto the saw horses. The chalk line should not go over the saw horses.
Wearing safety glasses, cut along the line with a circular saw so that there are two equally sized pieces. Repeat these two steps with the other board.
Position the four boards on edge, 10 inches apart, with the hypotenuses of the triangular boards on top and aligned. Lay a sheet of plywood over the boards so that a 4-foot edge is flush with the back corner of the ramp. Use screws to fasten the plywood to the boards, spacing the screws 1 foot apart.
Place the second sheet of plywood on the boards so it is aligned with the first sheet and there is no gap between them. Screw the plywood to the boards in the same manner.