Funniest of questions. My answer first block diagram,major part selection,know what signal to route,volatges required,real estate estimate of the parts,. Use layer and PCB thickness. And the company's old project uses them i will take one of them as they will be fabricated with a vendor for a long time. Then will come next question? Consider we are going to a new vendor? I was thinking what the heck man,then why did nt frame the question with "How will you choose a stack up for a new project to new vendor"? I had to answer that. Have they used Stack up planner in hyperlynx they can play around with impedances , Attenuation, and can use Power integrity tool to fix the prepregs height to fix it.. stack planning is not such an easy question based on thumb rules... And Black magic is not the guru of it