First have to drain it properly. Use garden hose or a submersibal pump. Once the pool is drained you can then flush the system. After the system is flushed, then securely attach a cover to the top and your spa will be set for the off season.
If your filter, pump and heater is in a freeze stiuation make sure you drain them and add RV antifreeze to the pump. Blow out any pipes with water. Add RV antifreeze to pool skimmers. Drain pool about half way and cover. You might add clorine to pool water.