It has been only recently that I have been able to balance in a headstand for a full 10 breaths.
Headstand or Shirshasana is one of the last poses in the finishing sequence. It takes practise and patience to finally achieve the full pose.
It is important to keep you shoulder and arms engaged, in order to keep your weight off of your head. You should be able to move your head side to side without difficulty.
To build up strength one can work in dolphin pose:
When that becomes easy, which may take weeks or months, then one can start laying the foundation of a headstand.
With your arms like you did in dolphin pose, hands clasped, place top of head onto the floor, with your fist behind your head.
Slowly walk your legs towards your body as much as you can. Engage your core.
Bring one leg to your chest - without kicking! Make sure there is no pressure on your head. If you don't have enough strength yet. Just keep practising with what you can do before attempting to do the full pose.
Next bring the other leg up. I normally rest in between each of one of these steps.
Finally bring both legs to your chest.
Lift each leg individually to the sky and balance.
Lift both legs next.