Consider it like flying a plane, literally. You'll need a straight section of tarmac to land on, you'll need to lower your altitude and speed, then get your gear down before touching down.
TL;DR, Try and land on the Pitioss landing strip for practise.
So let's tackle those things one at a time.
A straight section of road
It doesn't have to be super long, but it will need to be reasonably straight, as when you actually touch down the airship will wiggle, and needs to level out. Avoid any street lights along the roadside, you clip one of those and it's great balls of fire.
Lower your altitude
Push the stick forward right? Yes, and hold it, otherwise the airship will climb back to cruising altitude, especially if Ignis is driving. So point the nose towards the ground.
Top tip here is start your descent before you reach your straight section of road. Remember you want to land there, not start coming down at that point. Think about being low enough to get your rear wheels down. So line up and start your descent early, you can always pull up if you need.
Lower your speed
Again, hold the button down to reduce your speed or the airship will go back to cruise speed. Keep the button down until all the ailerons have folded away and you're on the road. This will stop the car driving into a barrier and great balls of fire.
When you see the prompt to land, press and hold the button. Keep it pressed till you're on the ground. Worst case in car mode, you'll park, so yeah, no fireballs.