Ensure the source of the flooding has stopped. If there is ongoing flooding outside, the water will just seep back in if you start pumping water prematurely.
Before venturing into the basement, turn off all power. Do this by switching off the main breaker. The combination of water and electricity leads directly to electrocution.
Connect a hose to a gas-powered pump and switch it on. A gas powered pump is safer to use because you won't have to worry about the electrocution risks of an electric pump.
Pump out the water in increments. Pumping out water too fast can lead to structural damage. It's best to pump the water out over a couple days. If you notice the water rose since the previous pumping, it's because the soil is still water laden outside and water is still seeping in.
Finish pumping the water and then figure out where the water came in. Seek to resolve this problem before the next rain.