My stud girth, and many that I have seen, have a metal loop on the front/in the middle (like alarge version of the d rings on a saddle) that you can clip a martingale to if you have one that has clips at each end. Mine also has a bit in the middle with a strip of leather attached to the girth that you can hook your martingale through, and it then does up through a loop and onto a billet fastening (sorry if that's not very clear!).
If yuo don't have either of those options you may need a different girth or a different martingale!! Sorry!!
If you don't have a clip on your martingale can either use a carabiner as Kerilli suggested or, get a trigger clip, cut the stitching on the loop on the martingale, put the clip on and then fold the leather back and put the keper over it. If that makes sense?! I asked my saddler re stitching it up again but this is what he advised me to do. I used one of these clips:
my stud girth has a big D ring on it for clip-on martingale straps...and a leather strap to loop round the martingale and then buckles up...if that makes sense?!
Mine is like Subens martingale, so I just put my leather strap through the martingale loop and put it back through small loop on the girth. I dont bother doing up the bilet thing as its so fiddly but its never come undone...... yet...