Position the wrapped magnet on the inside of the aquarium glass pane and keep it in place with a counter-magnet on the outside.
Cover the counter magnet with a cloth as well, so you don't scratch the glass!
Now, carefully move the magnet on the outside.
The magnetic cleaning cloth will, of course, follow the movement.
That's how you can remove dirt and algae on the inside without any difficulty.
It leaves the aquarium perfectly clean!