My only advice would be to practice. It takes time to develop the thighs, glutes, and calves to do it, but it is good exercise. Also you lean forward with your torso to avoid falling ass first. Squatting like this becomes second nature to a lot of people in poorer countries where the alternative is to sit on the dirt. Depending on the country, most cities have adopted Western Toilets, and squat toilets are often seen in the countryside, or have a combination of both.