This is how we cut round drywall beads that meet in a 90 degree corner. We are going to be using a tool called a "Mider" to mark our lines so that our corners have no gaps where they meet. If there's gaps between the two beads, it's more likely to crack there. You can pick up a mider tool at any how to drywall
store. It's real simple to use, the outside edges are for the 90 degree corners. Use your pencil to mark where the bead needs to be cut. Cut the meets first, make sure they meet good, place them on the wall to see how they fit, and then you cut the lengths. Always cut the length after you have cut the round, because sometimes your have to adjust the meets and cut it a few times shortening the bead. Good rule of thumb is to always cut the bead a little long, and then shortening it a little after, it's easy to cut more, hard to stretch a bead tho ... When you cutting the bead make little cuts with your scissors, don't try to cut to fast or too much at a time. When we are done cutting round beads with the mider, we will proceed to apply the beads to the wall.