Grafting aka Kitchener Stitch is used to seamlessly join two sets of 'live' stitches together. It's commonly used at the toe of socks knit from the top down, but it's useful for other types of project too, including cowls! Two of my cowl designs, Folia Loop and my new Aether Cowl, are knit flat and then grafted garter-stitch-style.
Grafting garter stitch is a little simpler than the better-known method for stockinette, since in this case the steps for the front needle and back needle are identical.
When your project is ready to be grafted, prepare by breaking your yarn, leaving a long tail, then thread it onto a darning needle. Unpick your provisional cast on and place the live stitches onto another knitting needle. Ensure you have the same number of stitches on each needle. Hold the needles parallel, tips pointing to the right, with the wrong sides of the cowl together.
Begin grafting by inserting the darning needle into the 1st front-needle stitch purlwise, then into the 1st back-needle stitch purlwise. Now work garter-style Kitchener stitch:
*Front needle: insert darning needle knitwise into 1st stitch and slip the stitch off, insert darning needle into 2nd stitch purlwise, pull the yarn through, and adjust the tension. Back needle: as for front needle. Repeat from * until all stitches are grafted.
Tip: when adjusting the tension of the graft, I like to hold my left index finger under the grafted stitches so I can check that the graft matches the surrounding fabric.
Finally, break the yarn if necessary and pull it through the final stitch to fasten off. If you blocked your project before grafting and you feel the grafted row could do with blocking too, you can now spot-block it using a spray bottle.
For tips on casting on for a cowl which will later be grafted, see my previous tutorial: How to work a Provisional Cast On.