I'm still fairly new to the game myself but I have a few tips.
- Start at day/mission #1. You can earn way more money that way than the game gives you for skipping.
- Be sure to take down the carriers and aerodromes before they can spawn too many drones. (I think there's an enemy cap though?).
- Don't destroy the mission target until you've done all you want, or else you can't land to heal.
- There are sometimes friendly airstrips on the islands. Useful for an emergency landing or just when the base is faraway.