This is a very basic embroidery technique that will serve you well. It’s sometimes called a french knot or a bullion knot. It’s a nice decorative touch. It’s also the absolute fastest way to tie off a thread when you’re sewing – just make the knot on the inside of the garment instead of the outside, and bury the threads!
This works exactly like the cheater-easy-thread-knoting-trick, but with fabric in the way. (More technically, the aforementioned trick works exactly like a bullion knot, but without the fabric.)
I spaced the knots above by taking stitches on the underside of the fabric. After each knot, I push the needle down right next to the knot, and push it out where I want the next knot. I thought I should mention that, because I remember when my great aunt Gin first showed me these knots when I was like, 5, I was completely baffled by why there was no string between them. (I come from a long line of amazing embroideresses, who are probably wondering what went wrong… Le sigh. I sew. I do not particularly embroider.)
When I need to tie off a thread that I’m sewing with, I push it through to the back of the fabric, and make this same knot. It’s brilliant. If I’m extra worried about strength (which is almost always), I make a second right next to the first. You can tie off a thread in about 3 seconds.