I usually start a presentation design project by digesting all the available information, listen to a verbal version of the pitch, Google for market and competitor information, create a slide template based on a straightforward slide (the profit and loss account for example), and let the whole thing cook in my mind for a while.
I know when I leave the “cooking” phase when I am able to draw up the key idea of the presentation in one slide. That one takes a long time to design, but when it is done, all other slides follow really quickly.