Okay.. Lie on the floor on your side, so that your lower leg is a bit further forwards than the one on top. Lift the top leg up and place the sole of your foot behind the lower leg's knee, so that the upper leg's knee is pointing upwards. Place your lower arm so that elbow is the furthest point of your body and your head is resting on your hand, to keep your head up, still facing towards the side (i.e. keeping you lying on your side rather than on your back), not up. Then, while keeping the lower leg straight, lift it off the floor towards the ceiling about 10-15cm. And then lower it back down again. Repeat in intervals, you can feel how the inner thigh muscles make the leg rise. You can adjust your body position to what feels the most comfortable for you, as not everyone likes keeping the body absolutely straight, but it is the easiest to explain that way when one can't actually show what to do.
You can add more resistance by either lifting your leg a bit higher/slower or maybe attaching a small weight on your ankle, or lifting in stages (first 5cm, short hold, another 5 cm and short hold then another 5cm and then slowly lowering the leg back to ground) Remember to do all the movements slow to really make the muscles work and spare the joints, not just yank your leg up and drop down. And then switch over to the other side.
Nina x
Ps. Would recommend doing lots of other excercises as well, warming up the muscles before doing this including walking, and do remember to stretch the muscles you used after every workout!