The easiest way I can think of would be to achieve the same orbit as the thing your are trying to dock to, very similar to dock to something orbiting Kerbin. Use RCS when you are about 50m from the object and at a releative velocity of 0.0m/s (+/- 0.1m/s) as usual. It's difficult to say something more specific without seeing the orbit of the rogue object, can you post a solar system picture with the orbit perhaps? Naturally it would seem that it would be best to catch the rogue while close to Kerbin but I think that is much harder.