Skim off whatever algae you can from the pool using a pool skimmer. Scrub the inside edges and walls of the pool with an algae brush to dislodge stuck algae.
Purchase sufficient amounts of algaecide, as noted in the instructions on the bag, that match the total gallons of water in the pool.
Turn the pool pump on -- the power switch is located on the electrical control panel that is in the pool shed at one end of the pool. Adjust the speed of the pool pump to maximum -- this involves turning a knob or pressing a switch. Wait 10 minutes before continuing.
Stand at the deep end of the pool. Empty the bag of algaecide into the pool, near the inside edge of the wall, as you walk the perimeter of the pool. Continue this procedure until all of the algaecide has been poured into the pool.
Repeat this procedure, only with the shock treatment powder that was also purchased at the same time as the algaecide, and whose total was determined from the chart on its bag.
Leave the pool alone for 24 hours. The brush the inside edges and walls of the pool with the brush in the same manner as you did earlier. Skim algae off the surface of the water with the pool skimmer. Turn the pool pump off.