It's fairly straightforward, on the External interface, put a Bandwidth Pool for the outbound traffic and a Download Throttling rule for the inbound traffic.
In the bandwidth pools, you can set a limit, not just guarantee certain bandwidth. In the traffic selectors is where you identify the from and to and the service. Change the internal to external with the drop-down box.
Cheers - Bob
Sorry for any short responses. Posted from my iPhone.