Float Like A Butterfly, Sting Like A Bee

For my personal project, I would like to learn how to do the butterfly stroke when swimming.  I find it hard to do the stroke because I don't have much upper arm strength to do the circular motion and to pull myself up and forward.  I want to learn how to build that upper arm strength and learn how to endure it as long as possible.  I used to go swimming but I'm curious becasue I usually never attempt to do this stroke cause it requires too much strength and concentration.  But otherwise, the other strokes such as the freestyle (or crawl), backstroke, and the breast stroke.