I presume you are referring to monopodding off the magazine? This is easy - just rest the mag on the ground (or whatever other firm surface you are resting on), allowing the weight of the rifle to be born entirely by the mag, and then put your support hand wherever is most comfortable for shooting AND transitioning across the array of targets with BOTH your elbows down on the ground. There will only be one place for your elbows... wherever they naturally fall is where they should be, and this will likely vary somewhat depending on the specifics of the position and the length of your magazine. I find a 40-43 round double-column magazine or a 60-round Surefire works best for me, but the 30s can be workable if a tad short... forget it with 20s unless the shooting position is very uneven.
Bear in mind that one or two unenlightened matches consider monopodding off cinched mags to be equivalent to a bipod (
) and will bump you to Open, but this is really in the minority.
If you want to see examples, watch any of the many many YouTube match videos to see how the top guys do it.