There are lots of projects where I've said to use something called a "stretch stitch".
This type of stitch is necessary to use when sewing with a knit fabric at a point where it needs to stretch, like a waistband of a skirt.
Below, the stretch stitch is #1:
The regular running stitch is #2 and #3 is a long running stitch I usually use for gathering fabric. I don't know why I included it. I guess just to look like I know what I'm doing.
Anyway, here is my machine. It is set in this picture for a basic non-stretch running or straight stitch. The green arrow shows the length of the stitch. The gray arrow shows what stitch I've selected. The yellow arrow shows the corresponding length to set the knobs to for the stitch I've selected.
To select a stretch stitch, I set the knob to "Stretch Stitch", shown by the blue arrow. That tells my machine to make the stitches at the bottom of the stitch selector, shown beside the red arrow. That first one is the one I use most often. I've used the zig-zag stretch stitch, too.
Hope this helps. I know everyone's machine isn't the same as mine, but it never hurts to have a visual of what I'm referencing. I'm no expert when it comes to sewing, but I have done a lot of trial and error. Mostly error.