If a pool has been left for a long time with standing water and it's too dirty to clean with chemicals or you have a leak that needs to be fixed, you will have to drain your pool. Draining a pool is not a hard project but can take quite a bit of time depending on the size of your pool. So make sure you have a weekend free of rain to drain the pool and do the work you need to do.
Place the sump pump at the shallow end of the pool. Place the drain hose in an area that will not be adversely affected by the amount of the draining water.
Plug the pump into an outlet that is high and away from the water. Allow the water to be pumped out to right above this area and then move the pump down to a deeper level. As it drains, keep moving the pump down until you get to the deepest spot in the pool.
When the sump pump gets all of the water out that it can, unplug it and take it out of the pool.
Plug in a shop vac and secure the plug from falling in the water by wrapping it around a heavy chair or taping it to a wall or chair arm.
Take the shop vac in the pool and vacuum out as much as it will hold. Empty the container and continue until all of the water is out. This goes much faster if you have help on the outside of the pool that you can hand the vacuum container to when it is full.