This is one of those things that you wouldn’t know about unless you’d had someone show you, and if you already know how to sew, you’d forget that beginners don’t know!
Your sewing machine should have a button or a lever (it’s different from machine to machine) that can make the machine stitch backwards. This is perfect for securing the ends of seams so your threads don’t just unravel.
On my newer Singer, it’s a large button/bar that you slide down, and it’s labeled with a little symbol that looks like a U-turn. On my vintage Kenmore, it’s a button with a big “R” on it (reverse). On my vintage Singer, the stitch length lever pushed all the way to the top position is reverse.
You’ll want to backstitch at least twice to make sure the seam is secure – that means when you get to the end of your seams, you’ll go backward, forward, backward, and then forward again.
For the purposes of the diagram, my backstitching was kind of messy- I wanted to show each back-and-forth motion. You want to try to line up those stitches on top of each other as best you can.