Mark the spots on the ground where the stair posts will be situated. Use spray paint or stakes.
Call your utility companies. Ask if electrical lines or pipes are buried in these places.
Dig holes for the stair footings at the places you marked. The holes should extend more than 6 inches below the frost line. This will prevent frost heaves from shifting the alignment of the posts. If the ground does not freeze in your area, then the footing holes must extend at least 18 inches below the surface. The holes should be at least 12 inches across.
Pour gravel into the holes to a depth of 6 inches. Do not exceed 6 inches. Pack the gravel down, using a length of 2-by-4 board.
Mix up a batch of concrete according to the manufacturer’s instructions. For such a small amount, hand mixing in a wheelbarrow will be easier and cheaper than using a cement mixer.
Set one of the posts into one of the holes. Pour the concrete into the hole around the post. Use a level to make sure that the post stands perfectly plumb.