Primarily I'd suggest loading up the main campaign maps and inspect stuff, get ideas from there.
Basically what I do is I play the game, see things I like and go "Hey, this is cool, how'd they do this?" then I go there in the editor and inspect. After that, I decide upon an atmosphere presert for a part of the map, edit the atmosphere if I want minor adjustments, then add pointlights or spotlights (Or spotlights with volumetric turned on through windows/cave openings) and such.
One thing I've noticed in the campaign is that there's rarely any location without some RS3 effects going on, wind, dust, water drops, falling leaves or some sort of lighting to spice up the scene.
It's all up to taste and your decision for what looks good and works for your goal :P
