I often use a circular needle when knitting big things, but when you knit up all your stitches you simply turn the whole thing over and start back the way you came....just treat the circular needle as a straight needle. In this way you'll be knitting back and forth on both sides ....after all, garter stitch is created when you knit every rows...2 rows create one garter. That's what I'd do...just knit back and forth till you get to where you wish your neck hole to be...then decide how many stitches it will take to make a decent sized 'head hole'.....subtract those from whatever number of stitches you have on your needle. Divide that number by 2 and you'll know how many stitches will appear on either side of the neck hole....so, knit those stitches and then bind of number of stitches for the neck hole....then knit the remainder of the row.. then knit to where you bound off and create, in whatever way you choose, the same number of stitches you bound off, then finish knitting to the end... you now have a neck hole in the middle. Then, knit back and forth down the front till you have the same number of garters you have on the back....bind off and you're done.