Once you have determined what height and space your stairs should occupy, you can cut out the pieces so that each piece will stack on top of the lower layer. For custom shapes, like the semi-circular stairs I show in the example, use a plate, or a large compass, or pencil tied to string which is pinned to the centre of your foam, to draw the curved edge.
This system of stacking your steps means your lowest step will be the total length of your staircase, the step above that will be the lowest step length, minus the run of the lowest step and so on. To make sure you don't make a mistake, cut your lowest (longest) step section first, and measure each successive cut against the one just before it, to make sure your steps stack up properly.
Try to keep the foam edges as straight as possible, this will make the process of covering your steps with air dry clay much easier.