First, tie an anchor bead (or just a big knot) to a long string. I used plain sewing thread, though you can also try embroidery floss if you want something super sturdy.
The amount of ribbon you'll need really just depends on how long your lei is, how long your needle is, and how tightly you squish the loops together. A general amount would be about 50-60 yards of ribbon (total).
I haven't tried it yet, but I think adding small beads between ribbon loops would probably help reduce the amount of ribbon used.
Anyway, I hope this helps a little! Best of luck with your lei!