Clean the steps completely. Sweep, dust, wash and dry them completely before proceeding.
Measure the width and tread (the length from the back of the step to the front) of the step and cut each tread with a utility knife.
Cut the tread a few inches less than the width of the stair so you can leave a bit of space on each side. Assume, as an example, that the stair measures 36 inches wide and you want each tread to measure 30 inches, leaving 6 inches total. Divide this number by 2 to determine how much space to leave on each side. In the case of the example, make a mark 3 inches from each edge of the stair.
Test the tread on the stair while it's dry to ensure it meets your specifications for size. Make any necessary adjustments to each tread and cut to size.
Spread contact cement along the bottom of each stair tread and press firmly down on each step from front to back.
Roll from side to side and front to back with a roller to ensure the tread is pressed completely down on the step, eliminating any air pockets.