Store an eye dropper with your emergency supplies, to be used only for this purpose. Mix thoroughly by stirring or shaking water in a container. Let the water stand for 30 minutes. A slight chlorine odor should be detectable. If not then the dosage should be repeated and the water allowed to stand for an additional 15 minutes.
Liquid bleach loses strength over time.
For this reason I mark each bleach container with the current date. Water purification tablets will purify one liter of water. The tablets have a shelf life of two years and lose their effectiveness if them get damp. Purify enough water to last a maximum of 48 hours. Water allowed to sit for longer than this may become re-contaminated.