Sit side by side with the student. You should be sitting in front of the green mat. This lesson can be done on a table or on the floor.
Place the brown blocks on the mat. They should all be facing the same direction so that the broad sides face one way and the narrow sides the other. They should not be lined up by size.
Place the largest block toward the top of the mat. Always grasp the block so that your hand spans the width of the block, as this holding pattern will help give a muscular impression of thickness when imitated by the student.
Line up the blocks to make a stair. They should progress from broadest and tallest to narrowest and shortest. Pause after each selection so that the student can see that you are carefully considering each selection, and make sure that you elaborately line up the prisms as you go so that the ends are all flush. Depending on what your Montessori school believes about the level of explanation that an instructor should provide to a child, you may do this exercise in silence, or describe what you are thinking as you go.
Have the child repeat the exercise. If the child begins to participate or direct you in selecting the next block and lining it up in the staircase, allow him to finish the exercise and complete the stair. If you finish the entire stair, then you will need to rearrange the blocks in random order again before inviting the child to try the exercise himself.
Encourage the child to repeat the exercise on her own on subsequent days. She will likely make some mistakes at first, but these are largely self-correcting since a staircase only results when the blocks are aligned correctly.