Start out with your bill. 

Fold the bill into an accordion shape starting on the short side of the bill, folding approximately a centimetre at a time.

Fold your accordion in half.

Using a tiny piece of tape, tape the ends together at the fold.

Open the other side of the folded bill and tape the two end bands around the center with ribbon inside the fold. 

Once you have them all folded and taped, you can string them onto your ribbon.

In terms of the length of the lei and the length of the ribbon. It can certainly vary. I like to start with a ribbon approximately four feet long. That gives you the ability to add other items to your lei and still have room to tie it.