Go as far into a sidesplit as you can. Make sure your hips and feet are in line with each other. There are various opinions on which way your feet and knees should be rotated, but toes up, kneecaps pointing up, heels only on the floor works for most people and is easiest on the knees.
Tense the muscles by trying to drive your feet through the ground. While tensing, do not support yourself with your hands, as doing so will take weight off the adductors and they won't tense as hard. You can even hold a weight to make them tense harder, but don't let the weight pull you down. Hold the tension for 5 seconds, then relax and increase the amplitude of the split. You may want to put something under your feet (newspaper, cushions etc.) to make it easier to slide out further.
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