The stem stitch is an embroidery stitch that’s similar to the backstitch in that it can also be used to embroider a straight line.
Fuse some lightweight interfacing to the back of your fabric to stabilize it
Thread the needle
Use 3 or 6 strands of floss, depending on how dark you want the line to be. Thread the needle and knot it at one end using a quilter’s knot.
To make a quilter’s knot, hold the threaded needle in your left hand. Pinch the thread under your left thumb. With your right hand, wrap the end of the thread three times around the needle.
Pinch the wrapped threads using your left thumb and forefinger, then gently slide them down off the needle and to the end of the thread. A knot will form at the end of the thread.
Bring the needle up from the back at #1 and down at #2
Start your stitching at the right end of the line you want to stitch. Then take a small stitch towards the left.
Bring the needle up from the back at #3
Notice that #3 is a tiny bit back towards the right, towards #1. Bring the needle up just below your first stitch (not through the stitch, but beside it).
Bring the needle down at #4
Take a small stitch to the left, bringing the needle down at #4
Repeat steps 2-4
End your stitching by tucking your thread under a stitch on the back of your work.
Outline vs Stem Stitch
To make an Outline stitch rather than a Stem Stitch, you follow the same steps but keep the thread above the stitching line.