When hanging drywall, it is always preferable to align the two tapered edges of adjoining sheets of drywall.
When you look at a piece of drywall closely and feel it with your hand, you will notice that on one of the long edges there is a taper. When two of these tapers meet, they form a void that is easily filled in with joint compound or mud and makes the join invisible.
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).
It 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.
The purpose is not to try 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.