Sitting on the floor with one knee bent, place the opposite ankle over the bent knee.
Place the tennis ball under your 'cheek' of the leg that is OFF the floor.
Then easing into the balls...breathe! You are in charge of the pressure. If it's too much then use your arms to take the weight off a little. You are looking for a little discomfort so if there are a few spots that feel uncomfortable (NOT painful) then stay there and breath.
You should feel the discomfort reduce. If it increases then move off and look again.
Stay on one side for around 60 seconds as you find a couple of different 'spots' to work on.
Repeat on each side 2-3 times.
Little and often is best but especially before activity and after periods of inactivity.