Combine the citric acid and essential oils (if using) and store in a jar. Place one tablespoon in your rinse aid compartment before running a load of dishes in the dishwasher.
You can also blend the citric acid with water and then add that to the rinse aid compartment, if you like, but it's really an extra unnecessary step.
Pitfalls of Peroxide Dishwasher Rinse Aid method.
Peroxide can bleach clothing so take care to not get this on your clothing.NOTE: Some recipes online recommend mixing peroxide with citric acid. Do NOT do that. Also don't mix peroxide with vinegar. Or with any acid. Mixing peroxide with acid makes a super strong oxidizer that can etch metal. NOT a good idea. (source)
Pitfalls of Citric Acid Dishwasher Rinse Aid method.
Citric Acid can get clumpy if you live in a humid environment. If this happens, you can place bentonite clay in a baby sock or small cloth, seal it with a rubber band, and keep it in the jar with your citric acid blend to absorb moisture.