Choose a 4-digit PIN and say it twice (you'll say it again, with a shown digit worked in, every time you sign in). How does this work?
One more thing: every time you sign in, Vokey shows you a random digit and asks you to work it into your PIN as you say it -- before, in the middle of, or after your four digits. Which position is used changes automatically on its own schedule, so it's never the same trick twice.
You're starting with: before your PIN (say the shown digit first, then your 4-digit PIN).