How do you do the butterfly properly?

I too have problems on the butterfly stroke. I tried doing that but only managed to do at least 5 strokes before sinking in the pool. I just don't know how you keep afloat with this stroke. It's frustrating but practice makes perfect.

Well I am sorry I do not know swimming.But swimming is a good exercise.

Butterfly is my slowest stroke but I'll try to describe exactly my coach taught me.
You need to increase your legs strength by imitating a dolphins kick, dont' move your hand yet just hold your arms forward, while you kick try moving your body with your legs exactly like how a dolphin moves its body.
Before your take-off or the part where you raise your arms above the water make sure that your hips are up and your shoulders are down, then move your arms backwards then up, don't ry to raise your arms too high as you might hurt your back.
Upon dropping your arms to the water, push your hips forward to raise your butt over water thus making your body move like a dolphin.
Practice a lot, if you can ack a coach for help.
I hope I helped a little.