Put a bit of scrap under the foot. Sew two short pieces: first by watching the needle, then by watching the seam guide on the throat plate. Both stitch lines may appear to be the same length, but the distance between stitch line and raw edge usually makes the difference. Seam allowance is more than the strip of fabric alongside the stitches; it affects the size, fit, durability, and how well a garment’s pieces match up.
A pattern is usually drafted with a specific seam allowance. If the seam on one side of a pattern piece is supposed to be sewn a certain amount in from the edge and you sew too far inside, the finished garment can be smaller than intended. If the needle moves out from the guide, the garment can become larger or lopsided. This can happen easily with a T-shirt or other basic garment, where you have two side seams, maybe a waistband and facing and sleeve opening to deal with, and the seam allowance adds up to a lot.
Many people learning to sew stare closely at the needle, because it seems like the place where the action is. But for most situations, the better target is the seam guide. Your hands position the fabric, keeping the raw edge lined up with that guide, while the needle does its thing. If you focus on the needle, the fabric can move without your noticing until the end.
Here’s another idea: Draw a couple of straight lines on a scrap of fabric, and sew alongside them using the same seam allowance you plan to use for your garment. Don’t push the machine. Keep the feed dogs moving and stay out of the feed dogs’ way. Don’t push the fabric, let your hands guide it. After you sew the seam, measure the seam allowance at several places. You don’t need to get it perfect; you just want to notice where your hands are moving the fabric from where the machine wants it.
Seams go more easily on straight lines, because you can keep the raw edge lined up with the guide. On a curve, you may not have a guide to reference, so go more slowly. Whether it’s an armhole or neck or pocket, or the hem edge or a circle or arc of any kind, the fabric edges can shift easily, and the seam allowance can twist. You can use pins or clips, or even baste, to help hold the pieces together. Work smaller sections of a long curve, then continue sewing more slowly and steadily to avoid stretching the fabric.
Pressing, too, is about seam allowance. After sewing the seam, press the fabric flat to let the stitches set, then press the seam open or to one side, depending on the task. You may have difficulty with a next step if the seam allowance is twisted and folded over, or if you’ve skipped pressing altogether. You’ll see the difference in bulky or bulky-avoided corners, seams and joins, especially after pressing the final garment.
A consistent seam allowance lets the whole project go easier. Pieces match up better on the garment. Hems are easier. The finished result is closer to pattern size. And fit is more predictable. Before you reach for the seam ripper at every “mistake,” measure the seam allowance to see where you need to adjust. Sewing again along the same line is easier than sewing again along the same line.