Flaps do not MAKE a plane fly slower, the ALLOW a plane to fly slower.

In other words, if a plane normallt stalls at 25mph, you need to approach at about 30mph and let that last 5mph bleed off as you flair (These are just approximate numbers)

The same plae with flaps will lower the stall speed to... Let's say 15mph. Now, you can approach at 20mph instead of 30mph

As for deploying them, I generally do it on the base leg. Bleed off as much speed as possible or the extra lift will cause the plane to balloon. In any case, the plane will usually rise a bit when they are first deployed. Just accept the extra few feet of altitude, and then fly the plane like you normally would.

If you over shoot the landing, just power up and go around, but only use about 3/4 throttle or the extra lift will carry the plane up like a homesick angel

You can either retract the flaps at a safe altitude and try again, or just leave them down and make another approach

You will soon get the feel for how much slower you need to approach for a nice landing.