It doesn't really make a difference.
The idea is that the PC is being described as doing a thing... and at some point, someone at the table notices that such description fits the trigger of the move.
Be it at the beginning, during, or end of the preaching activity, is irrelevant. Also because, most probably, the description will not be as lengthy and detailed as to encompass the actual speech given.
Fictionally it could be as brief as "and I ask them to remember their better natures".
Or it could be "and I spend the next week preaching to the people of the village about X and Z and Y".
I mean, practically it can't happen any other way... After a description is offered to the table then someone will point out that it triggers the Lead the Flock move, at which point the mechanics will take effect, offering the Priest a number of holds to be spent "during the speech". Which could look like a simple mention of what the audience does as a result of the speech... or could look like something more detailed, as in "and when I mention Gargaroth that's when they go into a frenzy".
It's up to the table, and set up in a way as to offer maximum narrative freedom, after the minimum requirement of "describe the PC preaching to a crowd" has been met :)