The French knot stitch is a great way to add detail and texture to your embroidery piece.  It can provide some very interesting effects to your work.  There are a couple of ways to make the stitch so I will demonstrate both below.

Method 1

Firstly, bring your needle up through the fabric where you want the knot to be.

Pull the needle all the way through and draw the thread through too, as shown below.

Wrap the thread around the needle twice.

Pull the needle and thread gently through the thread that you wrapped around the needle, keeping the knot that is forming close to the fabric using your finger as a guide. The knot should sit touching the fabric when all of the thread has been pulled. 

Push the needle back down through the fabric close to the knot that you have just made. 

You should then have a completed French knot!

Method 2

As before, push your needle up through the fabric where you want the knot to go.

Draw the needle and thread up through the fabric.

Push the needle back into the fabric right next to where the thread is coming through the fabric and bring it up again just the other side, as shown in the picture below.

 Before pulling the needle through, wrap the thread around the point of the needle twice.

Gently pull the needle and thread through the thread that you have wrapped around the needle, using your finger to keep the knot close to the fabric and to help to prevent it getting in a tangle!

The knot should sit close to the fabric with no gap in between.

Push the needle back in through the fabric right next to the knot that you have made.

 You should have a completed French knot that looks the same as the first, made by the other method.

You will probably develop a preferred method, finding one easier than another.  I, personally prefer method 2 as I find that I get in less of a tangle with that one!

You can make a slightly bigger knot by wrapping the thread around the needle more times.  I have shown this below by wrapping the thread around 3 times rather than 2.

      

As you can see below, this makes a bigger knot.  The bigger the knot, the untidier it can become though if not created slowly and carefully.  The knot length also increases though, which can make a different effect that can be used for making roses etc.  That will be in a future demonstration!  Have fun experimenting with different size knots and see what you can create.