Be seated comfortably with your spine straight, its ok to lean against something, the emphasis is to be in a relaxed easy posture without slumping the torso. You can be naked, wearing loose comfortable clothing, or if in a work suit you might loosen belts and ties, and after some practice do stage one in a toilet cubicle at work for 5 minutes on your break (yes really)!
Using whichever hand feels right, hold your sex centre. Depending upon your mobility shuffle fingers underneath your pelvis so you are sitting on them, tips of your fingers roughly around your perineum (behind the testicles for a man, just behind the opening of the vagina for the woman). If you can’t reach this far don’t worry! It’s more important to be relaxed in your body with the intention of scooping up your sex centre in a comfortable hold.