The procedure is very simple,
Once there Visa is confirmed then they have to go to NHA adjacent to Lulu centre in Alsaad and have to register themselves for the NHA license,
This has to be done with 6 months of there Visa confirmation.

They conduct exams in April, July and August. every yr.

If u need more info chk the site below mentioned
http://www.nha.org.qa/index.jsp

Hope this helps u