Drag out your buttonScreen Shot 2016-09-12 at 8.19.20 AM.png

Vertical Spacing to Top

Screen Shot 2016-09-12 at 8.19.53 AM.png

So now looks like

Screen Shot 2016-09-12 at 8.20.01 AM.png

Center horizontally by clicking Align button at bottom

Screen Shot 2016-09-12 at 8.22.16 AM.png

Screen Shot 2016-09-12 at 8.22.44 AM.png

Screen Shot 2016-09-12 at 8.23.03 AM.png

Run simulator should now get.Screen Shot 2016-09-12 at 8.23.32 AM.png

To hang a centered button underneath that drag out another button.

Screen Shot 2016-09-12 at 8.26.56 AM.png

Control drag from the bottom button to the top selecting Center Horizontally.

Screen Shot 2016-09-12 at 8.27.47 AM.png

Screen Shot 2016-09-12 at 8.29.39 AM.png

Then do the same control drag from the bottom button to the top again yet this time select Vertical Spacing.

Screen Shot 2016-09-12 at 8.30.07 AM.png

Screen Shot 2016-09-12 at 8.31.17 AM.png

Run simulator

Screen Shot 2016-09-12 at 8.31.42 AM.png