It sits on your cross bar attached by Velcro straps and strapped to the header stem.
This bike bag is from BTR Direct which is based in East Sussex, they do bike bags and accessory on line. link to the bag
This link has all the information you will need, while you are there check out some of their other great priced bags, as for the price.......... well impressed!
One of the Best price cycling accessories we have ever come across!
The bag has been used plenty of times now, it got a good soaking the other Saturday on a ride out, the contents were all kept dry, and the bag has kept its shape, it took 3 days to dry my shoes, so that is a idea on how wet the bag got!
Once the bag dried out, it was just a case of wipeing away the moisture from the phone compartment!
Zips are still going fine, no snags or catches.
Still recommend this bag as good cycling accessory for any bike trip!