Drywall corner bead usually is either metal, or a metal corner with paper. If you have a metal one, apply with drywall nails. If you have a paper one, put a mudbase down, and stick it into the mud base. One you've got that done. You don't true it up. You blend the whole thing in by using a long trowel. There is no absolute square, but you do have to get it embedded in mud.