Sitting cross-legged on the ground or floor is one of the most common positions for sitting meditation, also called zazen in the Japanese Zen tradition. There is no one correct way to do this since bodies and flexibility vary greatly.
However, a position is considered to be most comfortable and supportive for cross-legged sitting meditation. To achieve a grounded, stable position:
Try to sit cross-legged on the front third of a meditation pillow to raise your behind just a little so that the cushion acts like a wedge supporting your hips and letting your knees angle downward.
Do your best to ensure your knees touch either the ground or added support cushions. In this way you form a firm tripod base with your lower body.
Aim to keep your spine straight but not rigid.
You can also turn a zafu on its side, like a wheel, then place it between your legs and sit with your ankles tucked under you to meditate in a kneeling position. You can also meditate in a chair.
Shosan Vicki Austin of the San Francisco Zen Center has a video showing detailed instructions for optimal sitting meditation positions on the floor and in a chair. She emphasizes that sitting postures are adaptable to all body conditions.