Toilets typically use 1.5 to 3+ gallons of water per flush. Unless the toilet uses a special gray water system, it uses the same tap water as your sinks and showers.
To save some of this water, place your water bottle in your toilet tank. This will keep the bottle out of the landfill and save water every time you flush! This method reduces the amount of water your tank uses by taking up some of the volume. This is an idea adapted from the '70s, when folks placed bricks in their tanks, which is not recommended as it can damage the plumbing (http://consumerist.com/consumer/home/should-i-put-a-brick-in-my-toilet-to-reduce-water-use-267872.php)
I used pictures from this instructable: https://www.instructables.com/id/Conserve_Water_Convert_your_existing_toilet_into_/ as my camera is out of commission
Step 1: Fill the Bottle
First, fill the bottle with something that will make it more dense than the water in the tank. In other words, fill it with something that will make it sink to the bottom.
Of course you could fill it with water, which is okay, but in the spirit of conservation, you may want to fill it with other, more dense materials such as little tiny rocks, apples, or dare I say, a duck..
Step 2: Open Lid, Place Bottle in the Tank
Make sure the bottle does not come into contact with any of the flush mechanism. This may cause the flush to dysfunction, headaches, and general life complication.
Step 3: Close the Lid, You're Done!
This should not significantly affect toilet performance. It's just enough to save some water, which over time will add up to a great amount.