Erick,
Firstly, you have to enable caching in the settings interface in the StarLink application.
While I am still at home and have an active WiFi connection to my android tablet, I launch the StarLink app and I bring the map view to the fore. I then scroll around in the map interface to the place where I intend to fly. Just the act of viewing those areas causes the map to cache. Additionally, I will switch to satellite view and do the same thing, so both map and satellite images are cached.
Sometimes I will scroll around at one zoom level, and the zoom in and scroll around again in order to cache the maps and photos at different zoom levels. Then I turn off the WiFi conenction and head out to fly.
I have read somewhere that the cached maps "expire" after 30 days and also that if you forget to shut off the WiFi connection on your tablet and the tablet picks up a WiFi signal somewhere random while you're travelling to your flight location, that it can cause the cached maps to be over-written as the StarLink app will refresh the active map when it finds a new WiFi connection. I can't be certain if either of these two scenarios are true as I've not tested or experienced them.
Cheers,
Dan K.