Seriously. Write it down, as you go. Don't think "I'll remember that bit" because you can't be bothered to go and fetch your notebook. More than likely you won't. Ok, you might but I won't!
Bad photo with what is probably camera strap.
Actually, this applies to knitting up other patterns too. If you make changes or a stitch or row count is important make a note of it. Because by the time you have knit a foot and a second leg there is no way you will remember how many rows you worked on the last heel (yes you can count them but I always seem to miss one, or two).