How To Slip Stitch (sl st) = Insert hook in stitch, wrap yarn over hook, pull yarn through the stitch and through the loop on the hook, at the same time.
Below I give the abbreviated instructions followed by written out instructions, with extra explanation for beginners.
(double crochet, chain 2, slip stitch) in 4th chain from hook (this is the center),
Added Explanation: The directions which are in between the ( ) are all worked in a same place (in this case, worked in the 4th chain from the hook). Do not count the chain that is already on the hook when determining which chain is the 4th chain from the hook.
chain 3, (double crochet, chain 2, slip stitch) in center
Added Explanation: The directions which are in between the ( ) are all worked in a same place (in this case, worked in the spot which was previously indicated to be the center). In other words, after you chain 3, you will make a double crochet in the center, then chain 2, then slip stitch back in the center.