To begin working into Cossack Pose I recommend a gentle warm up such as a Sun Salutation. Then starting with the legs wide apart fold over from the hips.
Then bend your right knee and walk your hands across the floor over to right foot.
Soften the elbows and increase the bend of the knee to lengthen out the inner thigh on the left leg.
Walk the hands over to the left and repeat the work here.
Do this several times.
Come back to the right and now dig the left heel into the mat, increasing the inner thigh stretch.
Again repeat several times on each leg.
Then if you have the balance bring your hands to prayer and lift your upper body upright, as you can see It is still a work in progress for me!
Any questions please ask!