Perhaps use one of the paper bows to make a pattern? I've never seen this done with ribbon, but I can't imagine why it wouldn't work.

For those who are interested - I did find the instructions.

Take a 1 1/2" wide ribbon cut to 18".
Take a 1" wide matching/coordinating ribbon cut to 18"
Fold both in half. Place 1" ribbon inside 1 1/2" ribbon, matching center folds. Tack together at fold. Hand tack both sides of the wider ribbon together along edge approximately every 5". This will keep the smaller ribbon on inside. Pull on ends of small ribbon and the bow will form leaving tails to wrap around your package. I cut my bow ends at an angle so it would look nicer. You can do this with any size ribbon or any length (the longer, the bigger the bow)as long as the inside ribbon is smaller than the outside.