Unfortunately, it is not always possible to join a tapered edge to another tapered edge. Then, adjoining butt edges is your second best option. A butt edge is simply a 90 degree angle edge (one without a taper).
Here's the Tip: Slice Edge of Drywall Butt Edge Prior to HangingIt helps to take your utility knife and run it at a 45 degree angle down the entire length of each butt edge that will meet. No, you are not trying to create your own tapered edge; this is practically impossible to do by hand. What you're doing instead is slicing away edge paper.
When two butt edges are forced against each other, the paper has a tendency to ripple and buckle, creating an unsightly ridge. By slicing off about a quarter-inch of this paper, you prevent the two butt edges from creating this ridge.