1. First you need to cut your sponge the size & shape that best fits your aquarium, you can go for a corner filter...
(The sponge i used is just a regular cleaning sponge so I was able to cut it with scissors. You can also go for an 'aquarium sponge' that you can find at a pet shop, you might need to cut it using a bread knife. )
2. Cut a hole in the sponge to fit your large tube in it. You don't have to go all the way through. mid-way is enought.
3. drill tiny holes on one end of the large tube to let water in from the sponge.
4. Insert Tube in the sponge (tiny holes in)
5. Plug the air line tube to air pump and insert air tube in larger tube.
6. Voilà! Make sure you wash carefully all parts of the filter before moving it in your aquarium.
To fix it inside, you can either weigh it down with a rock or stick it to the side using a suction valve.