Draw a line from corner to corner on your 40-inch plywood and cut it in half with your circular saw so you have two equal-size triangular pieces. You can use the straight edge to ensure your line is straight. These are the sides to your dog ramp.
Position the sides of your dog ramp so they're resting on their 40-inch edges, parallel, 17 inches apart and so the hypotenuses (longest sides) are aligned.
Position five 17-inch boards perpendicularly between the ramp edges so their 7 ½-inch surfaces are flush with the hypotenuses. Space them 1 inch apart. Screw through the ramp sides and into the ends of the 17-inch boards. Use four screws for each board -- two at each end.
Set the last 17-inch board perpendicularly between the sides so it is on edge, and flush with the 20-inch edges. Screw through the ramp sides and into the 17-inch board using four screws.