Buy a reed trimmer. The first step in trimming your reed is purchasing a reed trimmer for the appropriate instrument. There are multiple different types of trimmers made for various instruments, including alto saxophone, clarinet, soprano saxophone and more.
Measure the reed according to the shape you desire. You can use a coin with approximately the same shape as the edge of the reed to remove some of the length off the end. Do not remove more than 1/32 of an inch, or 1.5 mm, as this will alter the sound of the reed as it is played in your instrument.
Trim the edge of the reed. You can do this by using your reed trimmer, or if you do not have one, you can hold a coin to the edge of the reed as you burn off the end, filing the edge once you have reached the length you desire.
Once you have trimmed the edge of the reed, shape it as needed. You can soften the sound by using sandpaper or a fine blade to shave off a layer from the flat of the reed.