You will need to make 26 7 yard-long ropes and 20 4 yard-long ropes.
SInce the 7-yard long pieces are the bulk of what you will need, let's start with them first.
First take two plastic grocery bags and cut them into thirds (check the photos above).
Unfold the center piece. This is your first strand.
Take the two side pieces and cut the handles apart and down the sides until you reach the bottom of the bag to make two long strands.
Repeat this process with another plastic bag.
Take four of the six strands and tie them together into a knot. Don't pull the knot very tight yet because you may want to increase the length of the chair later on, in which case you will need to take the knot out. One of the strands will be shorter than the others, but don't worry.
Spread out your four strands. Take the furthest left piece (red in the picture) and pull it over the inner left piece (yellow in the picture). Next, take the furthest right piece (blue in the picture) and pull it under the inner right piece (green in the picture). Now take the blue piece and pul it over the red piece.
Now start from the furthest left piece (now the yellow strand) and repeat.
(If you're confused, you can check out this video
to get the idea of braiding with four strands)
When you get to the end of a strand, simply tie on another strand and keep going. Try and braid in the ends of the knot as much as you can, but don't worry if they stick out. You can always cut the ends off later. Keep going until you reach 7 yards, but these do stretch so make sure that you stretch them a little bit before you measure!