TheoTown

It must be a line at least 12 tiles long.

You'll have to make a flat surface there. Airports are flat.

I mastered this myself today (Mally B on play store) .

Click a square to start (the black and white squares appear) then use one of the green lines either side to mark out the length and direction needed - the orange line just shows one edge of your intended runway, it is the green tiles that lay it.

If you wanted to make a short runway, clicking the "3" tile in either direction will provide you a 7×7 square airfield with a 5×3 runway. The square between your starting point and your selected "3" will be the central square of this entire space. Tiles that are 4 spaces and beyond can be accurately predicted.