Zippo lighters are a brand of lighters designed to be windproof and rainproof. Rather than simply letting out a stream of gas and lighting it with a spark, Zippo lighters enclose a fabric wick within a metal container; the wick stays moist with lighter fluid kept in a reservoir in a lower compartment. When a spark hits the wick, it burns in a way that is more resistant to wind and rain than regular lighters. This wick-based design means that occasionally the wick becomes frayed or too blackened to burn, however. Fixing this is part of having a Zippo lighter.
Hold the lighter firmly between the palm and lower three fingers of your off hand -- if you are right-handed this is your left hand, and vice-versa. Open the top of the Zippo with your thumb to reveal the wick.
Grip the wick with your tweezers or pliers close to bottom, where the wick feeds out of the lower compartment. Make sure that where you choose to grip the wick is not frayed or so damaged that it will break under strain.
Pull the wick upward gently but firmly. If you don't pull hard enough, the wick won't budge, but if you pull too hard, too much of the wick will come up. Pull until at least an inch or so of unburnt wick is showing beneath the burnt or frayed portion.
Cut the wick immediately under the burnt portion.
The lighter component of the Zippo sits inside the casings, which flip open or closed, and bear the decorations that many Zippo owners like. When you are pulling up the wick, use part of your thumb to hold down the lighter component so you pull up the wick, and not just pull the lighter out of the casing.
While exactly how long you want your wick to be is a personal preference, make sure you don't cut it too long. The longer the wick that shows, the larger the flame the Zippo makes will be.