You can do it several ways, i might upload a file untill tomorow if i dont explain good.
But i do it different ways, sometimes i add instance variables on the objects, called "Collision", 0 is false 1 is true. Also i prefer to control the overlapping points myself so i do invisible sprites that i call "enemycol" or something, sometimes its better to use the "on collision" , since its hard getting already overlapped things to not overlap again, especially if the object is not moving. You could use instance variables to detect collision, you will figure it out trust me, theres alot of ways!
basicly > if object1 collision with object 2 set instance variable "col" of object 1 to 1,
and do a invert event same as above but right click and click invert on the "is overlapping" , wich means you set the col to 0 cuz its invert, it means it only executes if its not overlapping.