I've had no success with ramps over the years but other members do and have done. As a general rule longer and shallower is better. Enclosed/raised sides are preferable and the use of a ramp tunnel may encourage them to use it. None of mine have used ramps so I removed the mezzanine levels in my cages, but I think it's just my particular piggies. @PiggyOwner has a great ramp and tunnel set up
I agree as less steep as possible for the ramp. I used a wooden ramp from a ferplast two storey cage in my c&c set up to get from one level to the next if that helps give you an idea. I have put a ramp tunnel on it to help me feel sure they won't fall off and they all use it happily.