Terraining is the funnest part of map making in my opinion. I dont even start on my first trigger till terrain is 100% complete, since setting up triggers often revolves around the terrain being complete, anyway. That way if my map idea doesnt work due to triggering limitations, I have the terrain ready to be recycled and for another map idea. Plus, It doesnt involve constant playtesting and tedious link clicking like triggerwork does.
I first do the overall layout of the map, or the "shape" so to speak. It really helps you figure out what you want to do with the rest because you then know exaclty how much space you have to work with and where. From that point on I start terraining it piece by piece, or I may jump around if I get bored in one section. Some doodads, like trees, really should be done across the map first since they are more limited in number and determine how many other doodads will be needed to go along with them (I'm one of those who always has rocks and grass around trees). And yes, if you ever plan to hace smooth shores along side water, you really do need to start with an all water map and raise the terrain from it.
-VGsatomi