To measure about how much hemp you will need, wrap the end of a strand around your neck (or wrist, for bracelets).
Cut the strand where it meets on the other side of your neck. You may want to add a couple of extra inches for good measure. Then double it over so you have two strands that make a loop in the center. This is your base strand that you will be tying the hemp knots around.
Pull the newly cut strand taunt and measure out another length of hemp next to it but don't cut the new strand. Instead, you will double the strand over and over again until you have 8 or 9 doubled strands.
After you have "doubled" the strands 8 or 9 times, cut the hemp at the end so that when you pull the strand taunt you will have one long piece of hemp. This will be you knotting hemp.