Hi Bro,
just to outline the basic process; 1st level is sandtrooper which is your 501st legion entry and approved by your local GMO. 2nd level is PO and 3rd level SWAT, both approved here at the MEPD. Each level requires that you have been approved on the previous one, i.e. you're not eligible for Swat if you are not a PO.
Hope that helps!