To stabilize your gun all you have to do it's simply find a flat surface, it can be a table, a wall, a vehicle or even just lying on the ground, once you found a flat surface take our your machinegun and hold right mouse button, the gun should snap to the flat surface making it stable, congratulation you just learn how to stabilize your weapon! (Notice: to stabilize your gun you're also required to have a bipod, most machinegun have them installed by default, but in chase your doesn't you should have the bipod for your machinegun in your inventory which you can just equip. Stabilization it's not an ability unique to machineguns, Anti-Tank rifles can also be stabilized)
Stabilize
If you're playing as an infantry man in the role of Machine-gunner you may have notice that your weapon has a lot of recoil, to reduce this recoil you can stabilize your gun!