How do you connect a copper water pipe to a flex pipe?

The quick and easy way is to use a compression "Shark Bite" fitting. It slips onto the copper pipe and creates a tight fit. Then use a threaded or pex flex adapter for your for the connection.

Sweat on a male adapter to the copper pipe then just screw on the flex hose. You can also cut the pipe sweat on a tee then use a nipple and male adapter.