To start open your template or skin and the file that contains markings like the ones which you can find here
In the file with the markings select the layer where the marking is that you want. In this example we want a USAF marking. If you are not sure on which layer your marking is, you can use the eye-icon in from of every layer to toggle the layer off and on. That way you can search for the marking.
Now select the lasso selection tool and draw a selectionline around the marking that you want. You need to do this to isolate the marking from the other USAF markings which are situated on the same layer.
Now copy the marking through the menu or by hitting CTRL-C.
Switch to the skin where you want to add the marking and make sure the spot where you want it is on your screen. I have zommed to 200% so it is easier to place the marking.