My descriptions are probably not the right way to refer to measurements of building materials, sorry!

Materials:
2 - 16 inch wide, 8 ft. long prepainted MDF Shelves
1 - 4 inch wide, 8 ft. long prepainted boards
2 - 2 inch wide, 8 ft long prepainted boards.
1/2 roll of 3M Tread Tape
2 hinges (if I was going to do this again, I'd use 4)
Screws

This is what I did:
cut all boards but the 1x2s in half.
cut one of the now 4 ft 16 inch boards in half again.
Lay 2 4ft 16inch boards end to end and the 24 inch board end to end with those - I screwed on hinges between the boards, so you can fold the boards. If I was to do this again, I would use 2 hinges at each connection instead of one - mostly for a little more stability - this thing is very heavy.

I screwed the 4 inch wide board alongside the board which will be the ramp (this would be the long board at the end of the hinged together boards)- to make a ledge to give the critters a little bit of assurance that they won't fall off.

Then John cut lots of 16 inch wide lengths out of the 1x2 board for me - I used them under the ramp board to make spacers where I can move the wedge board to raise and lower the ramp. On the top of the ramp, I alternated a screwed-in 2 inch wide (16 inches long) board with a piece of the stick on tread.

That's it.

If I ever make another one, I'll make the base shorter than the ramp by about 6 inches.