Premium cigars have a finished and closed head, which must be cut to draw properly. In addition to having a firm grip on the cigar and the cigar cutter, it is critical to cut the head/cap at the appropriate place. The head of the cigar has a convex, or curved shape, part of which is called the shoulder. A perfect cut will leave most of the shoulder intact. It is only necessary to remove a small amount of the wrapper to expose the filler. Cutting the entire head off will probably not improve the draw, but it can cause the cigar to unravel.
Regardless of the cutter being used (piercer, punch, guillotine, scissors), the main objective is to only remove enough wrapper leaf for the cigar to draw. Many cigars will draw well with just a little wrapper removed, while others may need a larger hole.
For the novice cigar smoker, an easy way to get a perfect cut from most double-bladed guillotine cutters is to lay the cutter flat on a table, place the cigar in it straight up at a 90° angle, and snip. This method makes it easy to not only get a straight cut, but with most cutters it snips just the right amount of the head off your cigar.