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This line should follow the natural line of your cheek. You want the line to be straight, starting at the bottom of your sideburn (point A) and ending at the edge of your mustache (point B). Identify specific points for A and B; don't generalize. If you're worried you can't create clean lines just by visualizing them, pick up a white pencil to draw yourself a guideline on each cheek. You can use a white eyeliner pencil, or hop online and order a barber's pencil. For a clean and defined line, don't use a guard for this. Carefully trim the hair above the visualized (or drawn) cheek line. Shave downward, with the grain of your hair, starting at the sideburn (point A) and working your way down your face right next to your mustache (point B). If you want a very defined and angular look, don't worry about rounding off the transition points at your sideburn and mustache. For a softer, more natural line, round off the edges at the point where the beard meets your sideburn, and again at the point where it meets the edge of your mustache. Always do one cheek first, and then refer back to it as you're working on the other cheek to make sure things are even. Avoid going back and forth from cheek to cheek, which can result in you trimming off too much of your beard. Make the sides as even as you can, but don't stress over absolute perfection. If you want to keep your lines defined and sharp, you are probably going to have to do routine maintenance and clean up every other day. If your hair grows very fast, you may even need to do this daily.
Visualize a straight line from your sideburn to your mustache. Draw a straight line from point to point with a white pencil (optional). Trim the hair above one cheek line with a trimmer or clippers. Round off the transition points if desired. Repeat on your other cheek. Trim your cheek lines every 1-2 days for the cleanest results.