How to get to Mutianyu from Beijing with public transportation?

I have already been on a guided tour to the great wall, but would prefer to a more flexible tour, where I can decide how much time I want to spend at the great wall.

Is it possible to get to Mutianyu from Beijing with public transportation? How long time will it take and what is the approx. price of it?

What is the best section of the great wall to visit from Beijing?