This is all about Mattress Stitch, for sewing up the cowl, if that is the option you have gone for.
This is a sewing stitch done with a wool needle, not a knitting stitch. It is useful for making a flat join between two ends of knitted fabric so that they lie – well – flat. I’m not quite sure where the ‘mattress’ comes in!
To start with, your yarn is already secured to the knitting after casting off.
Place the cast on and off edges of the knitted fabric alongside each other, how you want them to look when they are joined. (Remember, you can lay the knitted fabric completely flat and then bring the cast on and cast off edges to just meet, like a belt. Or you can add a twist by flipping one end of the knitted fabric, once, so you’re joining the top of the cast off edge to the bottom of the cast on edge. Either way looks great, it’s up to you!)
Thread the wool needle and make one small stitch across from cast off to cast on edge and back again. Do not pull this one too tight or it will bunch up.
Your yarn is now attached to the cast off edge again. Put your needle IN to the stitch you just made but on OPPOSITE side of the knitting – the cast on edge. Move your wool needle under the fabric on that same side and poke it up OUT of the fabric one stitch higher up on the SAME side of your fabric (the cast on edge). Pull the needle and yarn through.
Then do the same but the other way around – put the needle IN on the cast off edge at the top of the last mattress stitch you made on the OPPOSITE side of the fabric from where our yarn is attached. Move your wool needle under the fabric on that same side and poke it OUT one stitch higher up on the SAME side of the fabric (the cast off edge). Pull the needle and yarn through.
Keep repeating this until you have joined both sides of the cowl all the way up. Finish off with a double stitch and then thread the yarn along inside the fabric for a couple of stitches to weave it in.
You will now have a lovely flat seam, all secured, and you can snip off the remaining yarn.