Summarize this article:

To ensure that you measure PEX tubing properly and cut it evenly, straighten out the tubing. Fold the tubing upward in the opposite direction that it is curling. Lay it down on a flat surface to make sure it is as straight as possible. PEX piping should never be stretched tightly between fittings. Use a measuring tape to find out exactly how much piping you need, then add about 1 inch (2.5cm). This will leave room for the flexible piping to expand and contract. Having a clearly marked cutting line will make the process of cutting your PEX tubing simple and help you avoid mistakes. After measuring your piping, draw a line to mark exactly where you want to cut. Use a pen or fine point marker to ensure that the line will not fade or wipe off before you make the cut. Place the PEX tubing into the mouth of the tool where you want it cut. Position the exact line you wish to cut through directly under the blade. Hold it firmly in place. Grip the handles of your cutter tool firmly. Squeeze them together in a strong, smooth motion so that the mouth closes on the tubing, pushing the blade down onto it. Keep squeezing until the blade goes completely though the tubing, or until you have squeezed the handles as far as they will go. If you are using a smaller PEX cutter, the first squeeze might not make a clean cut. If this is the case, stop and allow the mouth of the tool to reopen. Squeeze the cutter again and twist it downwards slightly to finish the cut.
Straighten out the PEX tubing as much as possible. Measure your piping and add about 1 inch (2.5cm). Mark the cutting line clearly with a pen or fine point marker. Insert the part of the PEX tubing you want to cut into the mouth of the tool. Squeeze the cutter tool firmly for a clean cut. Keep squeezing and twist the cutter downwards to finish the cut.