Hi Tom,
While you have internet access to your device, begin by building your mission shape using a grid mission or maybe in your case, a linear mission. Don't forget to add an estimated takeoff location by double tapping.
When that looks good, open the third tab down on the left side of your screen. Next make sure you are using one of the offline map options by tapping twice on the third icon over in this tab (the native apple maps cannot be saved).
If everything looks good, you can tap the first icon in this tab to download and save a portion of basemap surrounding your planned mission. The terrain tiles will be downloaded as well.
If the mission location is in an area where some cell signal still gets through but is not that great, remember to turn off your cell data on your device before recalling the mission. Map Pilot will always attempt to download the terrain tiles when you use the terrain awareness feature. If signal is poor it will get stuck or error out.
Before I go to a job site, I always test my planned missions before I leave home. You can do this by turning off wifi and cell data and attempting to recall the mission. You can also test that the terrain was loaded by tapping the Terrain Aware icon in the top tab of the screen.