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There are actually a few different kinds of seams out there, the Plain Seam, the Overlock, the French Seam, the Flat Seam, and the Laped Seam. For our purposes, and the purposes of the patterns we are making, we are going to be doing the Plain Seam.
Most seams are ⅝" (1.5 cm) from the raw edge, although you might find a pattern calling for a particular seam at ⅜ (1 cm). Your sewing machine will have a strike plate with grooves on the right side of the presser foot. These will indicate these two measurements (sometimes, they might even be labeled!) If you line up your fabric to be flush against those lines, sewing a seam is pretty easy. If you have more than 2 guide lines on your strike plate, I would suggest centering your needle, then using the manual knob on the side to lower it, then use your seam gauge to determine which line is the ⅜"/1cm line and which line is the ⅝"/1.5cm line, then use a paint pen or other indelible marker to highlight each line for easy reference.
Step 1: Finishing before you even started
This might seem counter-intuitive, but the first step of doing a standard seam is to finish the raw edges for each piece. This is easiest to do before you've joined your fabric pieces, so I advise that, before you start stitching your fabric together, you take a few minutes to do a quick zig-zag stitch along both edges of fabric (separately) that are going to be joined. You want to do this as close to the edge as you can without the needle slipping off the fabric, lining up your presser foot to the edge of the fabric is a good safe way to do this. This will keep your fabric from fraying into the seam as you wash and dry it, just like we did when we pre-washed our fabric.
When you're done with the zig-zag stitch, make sure you set back to a straight stitch!
There are other finishing methods out there, but the zig-zag stitch is a fairly simple beginner's finishing stitch. Once you get more comfortable with your sewing machine, you might want to switch to an overlock stitch on your raw edge (if your sewing machine can do overlock, that is).
NOTE: There are a couple of instances where you would skip this step. If your finishing method is pinking, then you don't want to pink before you stitch because you might lose your matching notches. The other time you don't need to do finishing before you sew is if, immediately after you do your seam, there is a "trim seam" step (meaning you will cut away the allowance and the finishing stitch).
Step 2: Pin-Basting
The next part of making a seam is pin-basting your pieces together along the seam edge. With the rights sides of the fabric facing together on the inside (wrong side out), you want match any notches, then match your edges, and pin baste the edges flush.
|Left: Notches match with right sides together: OK!|
Right: Notches do not match with right sides together: No!
When you start out, you want to use your seam gauge to measure your seam allowance (⅝" or whatever the pattern specifies), but as you get a little practice under your belt, you'll get fairly good at eyeballing this. Don't forget to orient your pins so that you can pull them out easily as you feed the fabric through the machine!
Step 3: Sewing the Seams of Love
Once the seam is pin-basted, it's time to go ahead and feed it through the machine. Make sure you have your machine set to do a straight-stitch, and you want a "normal" stitch length of about 2 (although you might need to adjust this based on the fabric weight). Then, lining up the cut edge to your strike plate guideline, just feed the fabric through. For your first couple of tries, it's not a bad idea to check where the needle is landing with your seam gauge and the manual knob to make adjustments before you get the hang of things.
Make sure you fix your seam at the beginning and the end. To fix a stitch on a non-digital machine, simply use the reverse button on your sewing machine to re-tread your stitching at the beginning and the end by a stitch or two. This will keep your thread from accidentally pulling out by creating a knot in the seam at either end.
Step 4: Ironing
Now that you have the seam put together, you want to take your joined piece to your ironing board and carefully press open the seam, making sure not to stretch or pull the fabric. Give it a good blast of steam to reinforce to the fabric what you're trying to do, but otherwise, don't use the iron to pull the seam away from the rest of the piece.
Sometimes, a pattern will tell you to press a seam a particular way (toward the collar, say, or away from the sleeve). To do this, open up the fabric like you did at first, but instead of pressing the seam open, press it to one side or the other.
To practice a simple seam, all you need are scraps. You can practice a straight seam or on a curve by laying two pieces of scrap on top of one another (right sides facing in) and then cutting out a piece that is flat on one long edge and curved on another (sort of "loaf-shaped", or if you prefer, the "fabric taco."). With the pieces still together, cut out a notch or two on the edge you want to practice on, then take the pieces apart and start by pin-basting them back together. It's not a bad idea to also practice ripping out a seam.
When you've finished practicing on your scraps -- keep them handy, you can practice Topstitching and Understitching on them.