Put the one side of the blind on your leg. Grab the strings tightly. If the slats are still loose, this is the time that you can pull on the strings to tighten/raise the slats to the top so they won’t get damaged during transport.
You want to pull the strings toward the bottom of the blind, then start wrapping the strings around the headrail.
You see here that we went down, around the back of the blind, and back down around the blind to where we started.
It is easier at this point to slightly tug the strings to be a little tighter, then repeat wrapping the strings around the blind.