Just go to NHA Office located in Hamad Medical City, Al-Istiqlal Road, almost opposite to the Parade Ground. Ask for the forms from the reception...they'll guide you to it. Just fill up the forms and resubmit alongwith a Fee of QR 100/- and credentials of the applicant. They'll give you the next exam's schedule.
You can visit their website by clicking on www.nha.gov.qa