I make dog bows but I am not sure I can explain it.

I will assume that the ribbon is already "shaped" like a bow since it already had a rubber band around it. this is half the battle. Put the folded ribbon in your one hand and a small rubber band around both your thumb and index finger. Then I simply wrap the rubber band around the middle of the bow ribbon twice (or 3 times if it is a bigger rubber band) just like you would wrap a band around a ponytail.

There is another way, but it is difficult and time consuming and unnecessary. this way works fine. But if you want to try it, you would kind of strech the rubber band out and wrap around middle of ribbon like you would be forming the letter "U". Then you'd need to slip one loop into the other loop and pull tight.

I can't say that I descibed it very well, it is easier if you can see me do it.

Good luck.