It's been a while since I made it, but I believe you need to mark the actual stitches. In other words, you need something like a safety pin (ideally coilless, so it doesn't get caught up on the yarn) to hook through each of the stitches you want to mark. If you use stitch markers on the needles, they'll keep ending up out of position because of the way the increases happen (the increased stitches will be between the markers and the stitches you're trying to mark).
As you knit, you'll move the markers up every few rows, keeping them in that same "column" of stitches. You may find, though, that after a while you don't really need the markers anymore, because you can "read" the knitting and see where those stitches are. There will be a visible line formed by that stitch on each row, surrounded by the increases.
HTH!
