A simple way to straighten a crosswise fabric edge or to simply cut a piece of fabric perfectly on grain is by tearing it from selvage to selvage. While this technique is extremely convenient, it does not work for all fabrics thus it is important to do some pre-testing before tearing into the final fabric cut. Most basic weave cotton fabrics, as well chiffon and charmeuse fabrics, can be easily torn along the crosswise grain without causing any damage.
Tearing the fabric is a great technique to use as a preliminary fabric preparation step when aligning and cutting a garment's sewing patterns. Perfectly straight crosswise edges ensure that when the fabric is folded, it remains perfectly on grain throughout the pattern cutting process.
1. Start by slitting the fabric into the selvage, parallel to the crosswise edge. It may be helpful to slit lower so that you have enough grab on both sides of the tear.
2. Gently pull and tear the fabric from selvage to selvage. The fabric should rip easily and not cause any damage (such as pulled threads) within the body of the fabric. If you find that the fabric is difficult to tear and the edges are getting stretched out of shape, it may be better to cut the edges instead.
To cut perfectly on the crosswise grain, you can pull a few fabric threads along the crosswise edge to isolate a line. You can then use this line as an accurate cutting guide.