Hi and welcome to the board. Wonky strips happen when you fold the fabric multiple times and the folds are not perfectly parallel and the cuts are not 90 degrees to the fold. If possible, leave your fabric as it comes from the bolt. It will be about 22-23" wide. This is where your 6X24" ruler will work best. You need to make sure that your cuts are perpendicular to the fold. Place one of the ruler lines on the fold to make sure it's square and cut off the least amount of fabric to square it off. when you make subsequent cuts, make sure that you line up the ruler with the fold each time. Often, after a couple cuts, you may have to re-square your fabric by cutting off a sliver.