This is going to seem like a stupid question I'm sure, but I've tried to watch videos and everything and just can't seem to find the answer to this. I'm trying to put together a model for a base plate for a camcorder to attach to a bracket that I already have. It will consist of 3 rectangles of decreasing size, with a threaded hole on the bottom and a stud on top.

I've mocked it up in Photoshop (because that I actually understand how to use), but I just can't get the blocks to anchor themselves to the center point of the shape below in fusion.

This is my first foray into 3d modelling, so be gentle. Was thinking of rounding the corners of the various bits as well. not sure how much work that would be to do.

Anyway, any help would be appreciated.