Up until this point, you have been gathering a lot of information and sharing some of that information with colleagues. In this phase, you have to convince... and ask for executive buy-in.
Before knocking on the door of your executive stakeholders and sharing all of the information you've found, it's best to put together a presentation that explains website optimization and how it will help these executives reach their goals.
Once the presentation is finalized, it's time to begin setting up meetings with executive stakeholders and asking for their support.
In summary, by following this process to prepare and then gain the support of key stakeholders, your chances of starting to build a successful testing program, and culture around testing increase significantly. Your executive stakeholders can be your program's champion, so ensure that you do the correct amount of research, ask the right stakeholders, and then provide clear and open lines of communication about how individual test campaigns and aggregate testing is doing.
Good luck and happy testing!