Here is how you hang one button beneath another.
You have x2 constraints you need to fullfill: x and y.

It’s confusing because we are going to draw x2 verticals lines, but the first is actually an x constraint.

The first constraint we are going to add is going to center the button horizontally under the upper button.

Screen Shot 2016-04-25 at 10.39.41 AM.png

Which results in

Screen Shot 2016-04-25 at 10.39.49 AM.png

So this is an x constraint – even though it looks visually like a y constraint. It’s red because we haven’t told it yet how to hang in the y. So we are going to do that now by adding vertical spacing by control dragging from the button button to the subscribe above.

Screen Shot 2016-04-25 at 10.40.10 AM.png

Which results it

Screen Shot 2016-04-25 at 10.40.17 AM.png

Now all our constraints have been filled and we are good to go!