The way I’ve done it in the past for other types is to take a digital cockpit poster, paste it into something like PowerPoint or Visio and either draw the flows using arrows or numbers. Both are fairly straight forward, although I’ve found Visio is a far easier platform if you’re thinking of migrating the image into another document (i.e. your company manuals).
If you’ve got the time and inclination, you can also draw the cockpit in Visio, Photoshop, or something similar; although that can take hundreds of hours depending on the level of detail you want. Unless you also plan on using that image for something else (training, etc.) it’s probably not worth the time.