How do I install a floor drain in an above ground pool?

You don't, that would compromise your liner. Draining a pool is only needed once or twice like when a liner is replaced. You could use your filter and pump to drain only the first few inches but do not do this. You can create a siphon drain with a garden hose. Throw about 10' of garden hose into the pool and then lay the next 30+ feet out on the ground pointing toward the lowest spot in your yard. Take the end of the hose in the pool and hold it against the inlet with the pump running. This will flush water through the hose and when you see water coming out at your low spot just drop the hose, the whole pool will drain over the next 12 hours. Once the siphon is working shut down your pump.