Thich Nhat Hanh:
The most stable posture for meditation is sitting cross-legged on a cushion. Choose a cushion that is the right thickness to support you. The half-lotus and full-lotus positions are excellent for establishing stability of body and mind. To sit in the lotus position, gently cross your legs by placing one foot (for the half-lotus) or both feet (for the full-lotus) on the opposite thighs. If the lotus position is difficult, it is fine just to sit cross-legged or in any comfortable position.