How to File Your Nails Into a Soft, Even Curve
For a soft curve on a short fingernail, leave the middle of the free edge nearly straight. A few light strokes at the two corners usually do most of the shaping.
Find the two corners
Rest your hand on a towel and look at the nail from the front in even light. If a long nail needs shortening, clip almost straight across first. Leave enough free edge to keep the file off the skin.
Shape one side at a time
Set a fine emery board against one corner of the free edge. Sweep toward the center, lift the file, then return to the starting point. Repeat from the other corner. Keep the file on the edge and avoid sawing back and forth or digging into the skin folds.
Check the outline at eye level
After two or three strokes, put down the file and turn your palm toward you. Compare the corners of that nail. If one corner projects farther, work only on that corner. Each finger has a different nail width, so let each keep its own curve.
Know when to put the file down
Run another fingertip lightly along the edge. If you feel a snag, give that spot one light stroke and check again. If the file starts touching skin, stop and let the free edge grow before shaping again.
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Source · American Academy of Dermatology