The first thing you should do is closely examine the "head" of the cigar - this is the closed end that needs to be clipped. Almost all have what is called a "cap" - a bit of tobacco leaf used to close off the end - you should be able to see how far down the length of the cigar the cap goes by inspection. Typically only a 1/4" - 3/8" or so; sometimes much less, and on figurado shapes sometimes quite longer. Anyhow wherever the cap stops is your cutting limit - cut beneath the cap's line or even too close and your cigar will start to unravel.
Cigar Cutting Tools and Special Cuts
Knives: It takes a great deal of skill to become adept at using a knife for cigar cutting. However, they are easy to keep sharp, just don't clean your pocketknife with oil as it may damage fine cigars.
Single Blade Cutters: A guillotine cutter typically consists of a thumb-operated plunger, which slides a blade through a circular window in the body. The blade cuts on an angle like a true guillotine and slices off the head of the cigar poking through the hole.
A newer version of this cutter provides a much cleaner cut by using two opposing blades, which cut from both sides of the hole. Instead of a thumb plunger, the thumb and middle finger use loops for withdrawing and slicing with both blades.
One thing to keep in mind when using a guillotine cutter is to line up your cigar at eye level and clip quickly and decisively. This method gives satisfactory results most of the time.
The Pierce: This style is one of the more traditional cuts and can be used on any sized cigar. A piercer bores a hole into the head of the cigar. There is a risk of compressing the tobacco which, in turn, will interfere with the cigars drawing ability and cause overheating, thus affecting the taste.
The Bullseye A sharp, open-ended cylinder, which is placed on the head of the cigar and rotated. This will remove a round plug of tobacco. Stay within 1/16 of an inch into the head to avoid collapsing the head of the cigar.
There are also scissors (difficult to carry around and use) although elegant. Try a pair out before investing in them.
Choose whichever cutter or method of method of cutting that works best for you. The most important thing to remember is where the cigar cap ends; with practice, you'll be fine.