First, draw your circle on your fabric where you want your posy. For the purpose of this demonstration, I made an extra large circle. Ideally, you'll want the outside of your circle to be the last of your wraps. See below.
Next, you'll want to insert your pins into the fabric. The closer you space the pins, the smaller the circle. The wider you space the pins, the wider your circle. Again, for this demo, I spaced them wider.
Next, you'll bring your needle up to the left of the top pin as shown.
Now move your needle out of the way as shown and begin wrapping your thread under your pins as indicated. I went clock wise, but it would work well counter-clockwise too.
For the purpose of this demo, I wrapped maybe 7 or 8 times. Obviously, the denser you want the posy, the more you'll want to wrap. When you have made your last wrap, you'll take your needle down in the same spot you originally came up - to the left of the top pin. Ignore my pathetic nail. I've got to get a manicure!
Now, you'll need to secure your posy. You need holding stitches at each space between your pins. So bring your needle up on the inside and cross the wrap and take down on the outside edge as shown. You'll want to angle your needle slightly so it goes just under your wrap.
And this is what it should look like.
Then finish the other three sides. And this is what it should look like.
Notice that I did not take my posy all the way to my circle. That was done purposely for this demo. If you were to keep wrapping, you could go all the way to make a fuller posy. Remove your pins.
And this is the finished posy.
You can add french knots to the middle if you want. I added them to my daisy but chose to leave them plain for my day gown. It all depends on your project.