Flossing is a key to maintaining good long term gum health. It is very easy to do but there is a learning curve at the beginning. I have compiled two very useful videos that can demonstrate how to floss properly.
Watch how she is holding a floss. The length of floss should be about a foot and longer and hold it about a inch wide with your finger.
Note that the floss not only clean space between teeth but also inside gum space. Only up and down motion is recommended. Flossing left and right especially in back teeth can damage your teeth permanently and it is difficult to treat such damage.