Begin as you would for a regular chain stitch. Bring your needle up and then take it down very close to the spot you came up. Bring it back up a short distance along the seam line and wrap your thread around the needle before pulling it through. That gives you the first "link" in your chain.
Next, to make the tail or spine, put your needle down in a spot below and back a bit. How far depends on how large a tail you want. Bring your needle back up inside the chain, very close to where you started the tail.
You now carry on in the same way from this point, forming the next link in your chain.