The hardest part of restringing them is just getting the valance and lammykins off so you can get at the screws on the top of the blind set that holds it to the wall. If you're real lucky, you might get to them with a small ratchet with a #2 square drive bit in it, but that's a stretch.
You'll have to disassemble the two plastic sections for the day and the night portions to restring them, as well as the top header. Follow the instructions link that was given. You will need heavy string or just some 80lb. test fishing line. I suggest opening up the plastic sections before you start pulling string out, so you can see how it's laid in there, but it's not really complicated.
When one of mine broke, it was at the tension spring in the top header. There was enough excess wrapped around the tension knobs at the bottom to reattach it.