If you are new to sewing, and some or all of what I’ve been talking about is confusing to you or you just have no idea what I’m talking about, here is a bit of help: Here is the anatomy of a plain seam. The dark thread is a seam sewn with a straight stitch AKA a running stitch. For the purposes of this illustration, we’re pretending that we want some item that is x inches tall (the height of the photo) by y inches wide (the left edge of the fabric over to the seam) times 2 because our fabric was too narrow to make the y inches wide. So we add the seam allowance so our item will still end up y inches wide and not narrower. (If you want a 4 inch by 4 inch bean bag, you don’t cut a 4 inch square, you cut a square that is 4 inches PLUS the seam allowance on all sides. If it’s a 1/4″ seam allowance, your squares will be 4 1/2 inches.) Also, a plain seam is one that does not have the edges of the seam allowances finished in some way (with overlock, zigzag, bias tape, etc.).
Seam allowances can be different widths. Most commercial patterns have EVERY seam at 5/8 inch. In the industry, most seams are 3/8 inch with things like facings on necklines and the two sides of a cuff at 1/4 inch. I use 3/8″ and 1/4″ for the most part, but I’ve used 5/8″ for French seams and the part of the seam where a zipper is inserted. Coming up sooner or later 🙂 will be other types of seams. Please practice getting used to sewing on scraps of fabric. Start with woven fabric. Printed cotton is best because it’s easy to sew, doesn’t fray a lot, and you can easily tell which is the right side and which is the wrong side. Practice keeping your seam straight and the same width.