Start off by coming up through the fabric.
Hold the needle with your dominant hand. Wrap the thread around the needle 2-3 times depending on how big you want the knot.
Now, go back down the same hole you came out.
Push the knot all the way down your needle so it is touching the fabric. Take your time and gently nudge the needle through. You can keep the knot stable with a finger nail as you pull the rest of the tail through. That's all there is to it!
I love this example of what can be created with french knots from Incy Wincy Stitches.