When can I start massage my head after hair transplant done and how I do that massage?

Massage your head after asking your doctor if it is ok.  There is nothing wrong with massaging your head, but make sure your scalp has healed.  Always check with your doctor.

Until your scabs have come off, your transplanted hairs are easily injured. Two weeks after a hair transplant is generally safe, but most patients are advised to be gentle with the area for up to a  month.

You should probably wait at least 3 weeks after having grafts to have a scalp massage.  Massage the scalp but try to avoid things or techniques that scratch or abrade the scalp and newly grafted areas.