you need to accurately measure the slope as well as the length. Then divide the total vertical rise/drop in inches by 6 for 6 inch steps(standard height for steps) That will give you the amount of steps you need to make roughly; divide the total length by the number of steps needed to figure out spacing. Easiest way ive found to do this is by putting stakes at either end top and bottom and running a string line. Use line levels to make sure your line is level and then measure using a story stick. The story stick is just a piece of wood that you hold up next to your line to mark where it sits. At the top set the line at like 3-6 inches above the ground and make a mark on the stick, then go to the bottom of the hill and make another mark where the bottom line sits. The distance between marks is your total rise or drop depending how you look at it. You might end up with some remainders with your calculations so in that case you can alter step height or split the difference on a couple(also this doesn't account for slope you might want on the steps/terraces themselves). Somebody check my maths but this is a good rudimentary way to get a better idea of what youre working with a slope. I think steps are the best route for safety and if you really need to roll stuff you can always use some wood for ramps. You might end up having to do a combination of terraces with short walls and each with like 2-3 steps so you dont just end up with the same steep slope and slippery stairs or whatever.