Stack resolve question

[Jdogtoocool]

I have a {solemn simulacrum} and a {bitterheart witch} out in play and I cast {black sun's zenith}.  Do I get to choose which creature dies first and second and then do I shuffle the zenith back into the deck? Or do I shuffle the zenith shortly after activating it then activate creature effects?

[Jdogtoocool]

Cool thanks