Hi Even,
The first question for you on this one would be whether or not you would like for the wireless traffic to be centrally, or locally switched. When a WLAN is configured for Central switching (default), all traffic at the remote sites will be tunneled back to the WLC, and then dropped on the distribution network. When in local switching mode, the H-REAP feature is utilized, and client traffic is dropped at the remote AP, rather than being dropped at the WLC. I would recommend first deciding how you wish to handle the traffic, and then move on from there.
Here are some additional links that should help in your design:
H-REAP Modes of Operation Configuration Example
http://www.cisco.com/en/US/tech/tk722/tk809/technologies_configuration_example09186a00807cc3b8.shtml
Configuring Access Point Groups.
http://www.cisco.com/en/US/docs/wireless/controller/6.0/configuration/guide/c60wlan.html#wp1128591
Overview of RF Groups:
http://www.cisco.com/en/US/docs/wireless/controller/6.0/configuration/guide/c60rrm.html#wp1097383
Cheers.
Drew