Translate sentence by sentence, which makes it easier to understand:

You must take off from the airport first.

You must have flown from the airport,

After the library, to the central street,

Through the library, to the central street,

Go along this street and turn left.

Go along this street and turn left.

Then you can see a supermarket.

Then you can see a supermarket.

There is a post office opposite.

There is a post office on the other side.