I find that french knots need a lot of practice. Every time I need to do one, I do about 4 - 5 practice knots on the edge of my fabric and then do them where needed. This helps because you get into the groove of how to do them.
The trick is after you wrap the threa around the needle, pull it taut so that the knot is right against the fabric. And when you push your needle down through to the back, dont go down in the same hole otherwise the knot will end up in the back.
I think it is just a matter of doing it over and over again till you can. I was taught embroidery as a child, and that included French knots. I probably did thousands! Now I just do them, and cannot understand what all the fuss is about.
Sorry, not a lot of help, but the youtube videos seem good and straightforward.
chekccard