I would like to share a great tutorial for a square string, named also the Viking string.
This is the way how I like to make it, comfortably sitting with the string attached to my toe.
You need seven strings in two colors, 4 of one and 3 of the other, about a fathom (=6 ft) long.
Fold them in half and tie them together at the loose ends.
Now put them on your fingers: 4 on the left hand, on 4 of your fingers (not on your thumb), and 3 on your right one, on 3 of the fingers (the little, the ring and the middle finger).
With your right forefinger go through the loops on your left hand and grab the lowest string of the little finger. Pull it through. Now you have 4 occupated fingers on the right hand and only three on your left.
Move the strings on your left hand lower: from the ring finger to the little one, from the middle to the ring and from the forefinger to the middle one.
Now use your left forefinger and go through all of the strings on the right hand to grab the lowest of your little finger. Move the strings one finger lower and repeat and repeat.