Help With Ramps!

Okay... so... I have taken on the challenge of building my piggies a cage. It's a 2x4, two levels, with a bottom level for height. So... My "second story" is where the guinea pig living area begins, which is the general open 2x4 rectangle. I have two lofts, one on the right and one on the left, 2x1 lofts. So... my issue is I am not sure what to use for ramps. Does anybody have any advice on what I could use to build a good ramp for my piggies? Any advice would be greatly appreciated. Thanks!