You should first have a bunch of rows knitted. That's the easy bit. Next you knit half of the rows, turn the knitting, then knit them again. Knit those rows again, then turn the knitting and knit back to the beginning of the row. Do this a bunch of times, until the strip of knitting is as long as you want. There should be a poor, neglected set of stitches on one of the needles.
OH! I think I remember the step I forgot to include in the original keyhole instructions. (sorry...I didn't have the scarf in front of me...) Cut the yarn when you've got the Right (as in "correct") Side facing you. This would be either side, since we're doing garter stitch (knit each row).
Join the yarn to the end of the poor neglected stitches and knit as many rows as you had, then knit across.
Knit until you run out of yarn or until you've gotten to the right length.
Better directions found here.