How do you bridge a device?

Hi Guys

I have a dlink DSL 300g+ ethernet modem and am trying to add a router to the set-up but am getting no luck, I think when you try to add a router to this setup it is not straightforward as it should be. The dlink device needs to become a bridge for the router but I am not sure how to do this. I would be very grateful if somebody could give me an idea of how I can make the dlink a bridge device from my new router.

Thank for your help

Martin