Start with one of the 2.5" ribbons, leave a tail and then make one loop on each side of your bow. Take the second 2.5" ribbon and do the same thing. You'll want all of these loops about the same size.
Next use one of the 1.5" ribbons. Again, make a tail and two loops making these loops smaller than your last ones. Repeat with the next 1.5" ribbon and again with the last 2.5" ribbon.
Use a pipe cleaner or zip tie to secure the bow in place. Cut your ribbons of your spools leaving really long tails.