Cut strings to size depending on the amount of colors you're using. You'll want to start with white as your longest, getting darker and shorter, and finishing with a deep dark color. Your shortest color won't show at all.
Double forward knot your first color around each subsequent string. The full tutorial has more broken down steps for you.
Repeat with the next color, only don't tie it around the first string. Keep doing this with the next colors - don't knot it around those that have already been knotted
Reverse and do backward knots - and repeat with every color!