Here are four entries into headstand, for beginners to advanced level. Follow the same principles of the posture, which I outlined in my blog Headstand with Blocks, to set the foundation of the pose up.
1) Entering with Knees Bent: find your balance with one knee to chest, gently push your weight off the other leg, till then both knees slowly come to chest. From here, lift your knees towards ceiling, and move feet over your head towards the floor behind you. Lastly lift your feet up into the final posture. Come down the same way
2) Entering One Leg at a Time: Lift one leg up, keeping it very straight and strong. Keep lifting this leg towards the ceiling. Gently push off your foot on the floor, make little movements until you find your balance with both feet off, and at this point slowly raise the second foot up to meet your first foot.
3) Entering Lifting Both Legs Together: walk your feet in as close as you can, keep lifting your hips as high as possible. Push down through your wrists, lift your shoulders away from ears, and fix your shoulder blades. Lift both legs up. For flexible students, be careful not to arch your back, especially the lumbar. Keep the back of your body extended. Come down continuing to lift hips and shoulders up.
4) Jumping into Tripod Headstand: Mukta Hasta Sirsasana A is a headstand from Ashtanga’s Second Series, and takes some courage to jump into. Enter from downward facing dog, keep your hips and shoulders lifting, jump lightly and engage your core. Remember, your core is your back muscles (latissimus dorsi, serratus, etc – which you can engage with your shoulder blades), as well as your abdominals. Come down the same way, till it’s time to lift your head off – then let your arms take over and your feet land into chaturanga.