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Select a guard. Turn the trimmer on and with a gentle but firm pressure, trim each side of your face, working along the grain using long smooth strokes. Trim your mustache and chin. Finish the trim with a clean edge.
Most clippers come with several snap-on plastic guards. The guards ensure an even trim at the length you want - no one wants to cut their beard too short.  If you're not sure which of two guards is best, start with the longer attachment first. You can always go back over the beard with a shorter guard afterward, but if you trim too short, you can't put it back on. If you don't have a guard available, you can hold a comb in front of the clippers and trim off the hair that slips through the comb's teeth. Maintain balance between each side of your face by always starting at the ears and working down. The guard should keep the trimmer from irritating your skin or cutting too much hair. Start under your nose and move toward the corner of the mouth, then proceed down to your chin. Pay special attention to hard-to-reach areas directly under your nose.  Keep your mouth closed to avoid a mouthful of hair! Depending on the length of your beard, it may be easier to trim the mustache separately with scissors. Remove the guard from the trimmer and trim along your neckline to remove the remaining hair on your neck. Use your jaw as a straight edge. If you like, you can also lather and shave your neck, depending on how you like it styled. Some prefer a clean cut-off line for their beard, while others prefer subtle neck stubble.