Unless you have an automatic cleaner, an in-floor cleaning system or an automatic cover, or sometimes even if you do...you'll need to manually vacuum the debris. And here's how...
Roll your vacuum hose straight along the length of the pool. Attach one of the cuffed ends onto your vacuum head which is attached to your telescopic pole. Extend the pole and place the head (with the hose attached) into the water so that it rests on the floor of the pool. Point the head across the pool so that it doesn't roll down the slope towards the deep end and prop the pole up against the pool's edge.
From the point where the hose surfaces, begin pushing the hose straight down into the water, hand over hand, until you reach the other end. This is filling the hose up with water so there is no air in it which may cause difficulties for the pump when you attach the hose to the skimmer. Another method of "priming the hose" is to hold the cuffed end firmly over a return fitting to force the air out of the end attached to the vacuum head.
Once the hose is primed, remove the skimmer lid and the basket and stick the hose end into the hole at the bottom of the skimmer. If it sucks it in tightly, great. If not, you may need a threaded hose adapter to achieve a tight fit. Now, the suction that was at the hole is now at the vacuum head. Do not lift the head out of the water with the hose attached, or you will fill the hose with air, losing prime, and possibly draw air into the pump.
Roll the vacuum head on the floor, over the debris, and VOILA!, you're vacuuming. The suction will gradually decrease as the pump basket fills with vacuumed debris. When the pressure gauge drops and/or suction is sufficiently decreased, stop the pump and empty the basket. If the pressure rises significantly, stop the pump and backwash the filter. Continue in this manner until the pool is clean.