Choose which T-shirt or tank top you want to cut into a crop top. Especially on the first time, be sure to pick one that you won't miss if it doesn't turn out exactly right.
Decide how short you want the shirt to be. You can cut it just below the breasts, at the middle of the stomach, or right above the line of your jeans. Using a non-permanent marker, draw a line where you want the crop to be.
Lay the shirt out on the floor or on a table. Use a pair of fabric scissors to cut the shirt along the line at the desired length. If you plan to hem the top, cut at least half an inch below the line.
If you want a frayed look for your crop top, this step is optional. If you want a cleaner look, hem the bottom of the shirt.