just make it as you'd make difficulty normally expect you don't need to think about object placements at all. Only purpose of objects there is to be on sounds you want to have hitsounds.
So basicly in hitsound difficulty you just make a layer where you have those ticks covered that you want to hitsound and then add the samples you want to those objects.
You need to use external program to copy hitsounds to diffrent difficulties, one program i think most of the people use is mapping tools.