Sometimes it seems like the fabric is sliding off the line you’ve planned, and all you have to do is steer harder. That is when we tighten our hold, pull on the fabric behind the presser foot to make it do what we want, and end up with an even wackier seam.
The sewing machine has feed dogs that will move your fabric for you, so you don’t have to force it through. To get started, you need some scraps of cotton. Cotton fabric is easier to control than slippery materials like silk, or stretchy stuff like knits. Cut a few scrap pieces, make a couple of lines using a piece of tailor’s chalk (or a fabric marker that washes out), and sew those.
Draw a line closer to the edge if you want to practice the seam allowance, and one closer to the middle if you only want to focus on going straight. Before stitching on your fabric, make sure it’s flat, the presser foot is down, and your thread and bobbin are loaded correctly. When you are actually sewing, watch the seam guide on your needle plate rather than the needle itself. The needle is moving too fast, which will make your hands tighter. The seam guide is slower, and easier to follow. Pick a guide, place the side of your fabric up to that guide, and move it along that guide. The fabric can be touching the fabric guides, but your hands shouldn’t be moving it behind or in front of the needle.
One helpful exercise is to sew five lines without backstitching (this is something you will need to do once you are on a real project). Taking out that extra step will help you focus on the fabric movement. Sew slowly, stopping with the needle down if you need to re-adjust your hand placement. At the end of each line, compare your stitching with the edge of the fabric and look at where it started to pull. Look at how you are holding your hands or fingers, and if you were pulling on the side of the fabric near the end. If your fabric is moving around too much, check your needle first. You might have the wrong needle for the fabric, or the thread might be tangled, or your cutting might not be straight, or you might have stretched the fabric as you pinned it. You want your pins (or clips) to keep the fabric together with no bumps in the seam as you sew.
With fabric on top of fabric, place your pins perpendicular to your sewing line and remove them before they come under the needle. Or, use fabric clips along the edge that are out of the way of the sewing. You also need to slow down while sewing. If you go really fast, you might miss the moment the fabric starts to move off line. Slow sewing will give you time to notice the edge, notice the seam allowance, and notice how your fingers feel the fabric. Once you see that line getting more even, add backstitching at the beginning and end of your seam, and give it a press with your iron. The press will show you if the fabric got bunched, or stretched or just went smoothly. It’s okay if the fabric isn’t perfect; that is not the real measure of your progress.
You need to recognize when the fabric starts to move off line earlier, and correct with less force. As your hands get lighter and you can keep your eyes on your guide, your seam will look cleaner. The fabric will feel like it’s being carried, not like it’s being forced to do what you want.