Lie down flat on back and push belly button down towards the floor, your lower back should be touching the ground
Keep your abs and butt tight at all times, and with your arms pointed straight overhead and legs straight with toes pointed
Start slowly raising your legs and shoulders off the ground
Your head should come off the ground along with your shoulders, with your ears are glued between your shoulders
Stay tight with your abs and butt and find the lowest position you can have your arms and legs without them touching the ground AND without breaking your lower back (where it begins to arch off the ground)
To develop your hollow, you can start with your arms and legs higher (1-2 feet high off the ground) and slowly build up strength until they can be held lower (just inches off the ground) without breaking the position
Being able to hold the hollow body position is key in gymnastics, whether you're in a handstand, in a support hold on p-bars or rings, or doing straight and broad jumps. Continue to find and develop your hollow!