Cut three of the 72-inch boards diagonally, from corner to corner. You should have six equal-size triangular pieces. These will provide the slope for your ramp.
Lay three triangular pieces on their 72-inch edges, 15 3/4 inches apart, with the hypontenuse edges aligned (the hypotenuse is the side opposite the 90-degree angle). Lay a sheet of plywood over the hypotenuses so the edges are flush and screw the plywood to each board. Space the screws 12 inches apart.
Repeat Step 2 to create another ramp with the exact dimensions, using the last three triangular boards.
Set two 36-inch boards on edge, parallel to each other and 33 inches apart. Set three 33-inch boards perpendicularly between them so each of these boards are 15 3/4 inches apart.
Screw through the 36-inch boards and into the 33-inch boards, so you have a 36-inch by 36-inch frame.
Set the 36-inch by 36-inch piece of plywood on the frame so the edges are flush. Screw it down using 12 screws. This is the platform for your ramp.
Lay three 69-inch boards on edge, parallel to each other and 15 3/4 inches apart. Screw a 36-inch board to the ends of the 69-inch boards on both sides, using 12 screws. You will have a 36-inch by 72-inch frame.
Set a 4-inch by 4-inch board in both outside corners made with one of the 36-inch boards. The boards should be perpendicular to the frame. Screw through the frame and into these boards. This frame will support one of the ramps.
Position the 72-inch ramp perpendicularly against the porch, with the 4-inch by 4-inch boards next to the porch. Position the 36-inch by 36-inch platform against the 36-inch edge of the other frame.
Set one of the ramps on top of the 72-inch frame so the slant approaches the porch. The 4-inch by 4-inch boards should be inside the ramp. Screw through the sides and into the 4-inch by 4-inch boards. Screw through the low end of the ramp and into the frame beneath it.
Position the last ramp against the 36-inch by 36-inch platform on any side, to suit your needs.
Continue building frames underneath each other and securing them with 4-inch by 4-inch boards if you need a higher ramp. The same process is used for building the platforms.