ok it is hard to do. First right click on a toolbar. From the list pull up the selection filter toolbar and the quick snaps toolbar. Now you have snaps and selection of type of filter you need. Select what you are trying to snap.. being edge, vertex, faces, bodies etc.. now choose a quick snap function. The only one available is intersection of 2 curves. You can try also horizontal / vertical as well.
I can't be bothered using this method. The easiest way is to draw a few construction lines and snap to that. Painful I know, but easier.