I use a ramp with little 'rungs' or steps on it. The ramps are plywood over the top of supporting strips of lumber on each side.
The 'rungs' are strips of wood about 1x1 nailed or screwed every several inches horzontally across the ramp to act as foot holds for the chickens.
In my experience, the ramps can be flat or steep, long or short, but the shorter/steeper the ramp, the closer together the rungs need to be. Also, with a really steep ramp you may have to 'train' the chickens to go up. We would put the chicken on the ramp near the bottom and then use our hands to block the sides and sort of 'shoo' the chicken up the ramp a few times.
Once they figured out it was OK, they have no problems. Down was never a problem, BTW - it was always up that they were less than enthusiastic.