Step 1: Get a low-flow toilet, or reduce the water usage of your own
If you have a low-flow toilet (<1.6 gallons/flush), great, you're done. If you're not sure, check under the lid to see if it is listed. If you are looking to upgrade to one, there are many on the market that work well at below 1.3 gallons per flush, but they must be designed to do so (they usually work using higher pressures). If you upgrade, once you have it you'll be done with this guide.
If, however, you don't have one and don't want to invest in one, you can turn your own existing toilet into one. Go on to step 2 to find out how.