You can do this many ways. See the attached example for two different ways.
One of your basic choices is whether to use data dependency to control the program flow or a sequence structure. Either way, you need a while loop that stops when Start goes True and doesn't do anything else.
If you're using a sequence structure, put this while loop in sequence frame 0. Put initialization stuff in frame 1, then the rest of your program in frame 2.
If you're using data dependency, put some output you'll need for the next step (after Start) in the while loop and wire it to the next step outside of the loop.