Typically, with a sand or DE filter, when you put the filter on backwash it draws water in from the pool, through the filter, and sends it out to the drain hose (sometimes called waste). You can use that to drain the pool. If you have a main drain all you need to do is turn off the skimmers and the pool will drain from the main drain. If you don't have a main drain then you need to hook a manual pool vacuum hose up the a skimmer at one end and down at the bottom of the deep end at the other end and turn off any other skimmers. If you are doing this it is important that you not turn off the pump half way through or you will lose prime and not be able to get it back. Sometimes the vacuum hose leaks a little and that can sometimes cause a loss of prime. You can combat that by having a small pump or a garden hose feed a little water into the skimmer on top of the hose connection or vacuum plate. The water will seal any air leaks.
You should never completely drain an in ground pool with a vinyl liner (above ground pools are normally safe to drain). The liner will tend to shrink a little and you will be unlikely to get it back into the correct position and it can rip. When the liner is very new the odds of getting it back in place are quite good, but after a couple of months it becomes markedly less likely. There are techniques to hold the liner in place, but they are too complex to explain here and not something most people will want to try.
With a plaster/concrete or fiberglass pool you need to think about the water table. If the water table is above the bottom of the pool the pool can float up and cause significant damage to the plumbing and possibly crack apart. Usually the water table is below the pool and you will be fine. But if you figure wrong and the water table is high it can be very expensive to fix. Some pools have hydrostatic valves that equalize the water pressure, which means there can't be any damage (so long as the hydrostatic valve is still working). Others have well points around the pool. You can put a sump pump into each well point and pump the water table down to keep things safe.
