Knitting Garter Stitch is as simple as can be, but some people are confused when it comes to seaming Garter Stitch. It's not exactly like sewing mattress stitch on Stockinette, but it isn't any more difficult to seam up Garter Stitch for any project.
The main thing you need to understand is that Garter Stitch makes ridges and you need to follow the ridges to make them line up properly when you seam them.
Use the tail at the end of your project or attach a piece of yarn to sew with. Here I'm using a contrasting color but of course you would want to use the same yarn you knit your project with, or a coordinating yarn if your project is particularly bulky.
As with mattress stitch, you should only attempt this method of sewing Garter Stitch seams when you have two pieces of knitting that are the same length or when you are sewing two sides of a rectangle together, as with my Garter Stitch Armwarmers.
To sew mattress stitch or seam Garter Stitch, you work into the top of the ridge on one piece of knitting and the bottom of the ridge on the other.
I worked starting on the left-hand piece of knitting and put the needle through the stitch at the top of the ridge on that side.