Well, you will be using an access-list to achieve this. By using an extended access-list, you can specify the source ip address (the ip of the remote user) and the destination (your ip). Something like
access-list 101 permit ip host host .
This is then applied as an inbound access-list to the WAN interface.
As I'm not aware of your other requirements, I can not help you further with the access-list configuration.