need help with layers please

im not good with layers, like i cant put snow wher i want it
please help me out!

first put the tiles you want on a screen on a other map, place them exately where you want to have them on your screen, per example a tree tops you want to layer,go to the the screen you want to have the tree tops on, then go to layers you select the screen and the map where the tree tops are on,if you want to have it above link you must only select 3,4 or 4,5, not 1,2 that are the ones onder link, hope it helps