To make a French Knot, begin by threading the yarn through a tapestry needle. Bring the needle through the knitted fabric (not through a hole between stitches) from the back of the piece to the front (knotting the yarn in the back to keep it from coming through).
Take the yarn in your left hand and pull it taut off toward the left, while keeping the needle in your right hand. Wrap the yarn once around the needle, pull gently so that the yarn and needle are flat against the piece, wrap the yarn around the needle one or two more times, keeping even tension on it.
While continuing to hold the yarn, put the needle through the fabric from front to back, as close as possible to where the needle originally came up through (without going through the same spot!). Pull the yarn through the wraps to the back while continuing to hold the yarn taut. Once the yarn has been pulled all the way through, the French knot is formed.
Here is a free pattern where you can try out French Knots:
• Holiday Napkin Rings
• Child's Holiday Capelette