Add one quarter of a cup of works toilet bowel cleaner. (Or enough to fill about a half inch of the bottom).
Add water until 1/3 full.
Add about a teaspoon of dish washing liquid.
Add aquarium gravel until about a half inch is filled.
Shake, shake, shake, and shake some more.
Fill the bottle full of water.
Let sit 15 minutes.
Drain out.
Rinse out.
If the inside is just dirty you can use soapy water with gravel. Let it sit about 5 mins before you shake, so the dirt is loose.