Same logic and actions as backing a trailer (but with different responses): Just move the tongue in the direction it needs to go!!......just like if you were pushing it by hand. Anybody can do that right?
It only gets
really tricky when your backing a wagon up that's hooked behind another one, or behind a baler, chopper etc... Then I can only go a couple lengths before the S.H.T.F.
Obviously tongue length plays a part, just like with trailers. I was also going to say the turning radius of the vehicle you're doing the backing with plays a big part in how tight you can turn when backing, but I don't think this is 100% true. But it does effect how much space you need to get to and recover from the minimum turning radius that the wagon allows (if that makes sense).