Google Earth and lots of other good advice here.... but I would stress one thing: the best approach is to go to a location and scout it out. In the long run it will save you a tremendous amount of time on your flight plan.
Figure out the best takeoff spot and identify the tallest trees and structures relative to that spot so you can set an RTH above that. Check out the best angles, figure out where the sun is going to be, etc. Even if you just fly around a location for 15 minutes, that’s still a thousand times better (and safer) than relying on terrain data from Google Earth. Remember, the data set they pull from is pretty low resolution in most parts of the world (100 meters). And terrain data doesn’t give you the height of trees which is the real “ground” for a drone.
