Dishwasher tablets

Pressed tablets of washing soda, citric acid and salt. They're cheap and simple but may underperform commercial tabs, which contain enzymes.

Honesty note: these may underperform commercial tablets, which contain enzymes and surfactants. Expect good results on everyday loads and weaker results on baked-on food.
Washing soda is alkaline and citric acid irritates eyes. Wear gloves and a dust mask, and keep both away from eyes.
Never use these with chlorine bleach or a bleach-based dishwasher product. Citric acid and vinegar plus bleach release chlorine gas.
Tabs look like candy or sugar cubes. Label the container and store it out of reach of children and pets.
Don't use on aluminum pans, cast iron, gold-rimmed or hand-painted dishes. The alkaline mix can dull or strip them.

Steps

Put on gloves and a mask. Mix the washing soda, citric acid and salt thoroughly in the bowl, breaking up lumps.
Add the essential oil if you're using it and stir well.
Spritz a little water or vinegar at a time, stirring as you go. It will fizz a bit. Stop as soon as a squeezed handful holds together like damp sand.
Pack it firmly into the mold cavities, about 1 tablespoon each, and press down hard.
Let them dry for 24 hours in a dry room, then pop them out.
Store in a labeled airtight container.
Use one tab per load in the detergent cup. Use vinegar or rinse aid in the rinse-aid slot to prevent spots in hard water.

Tabs crumble: Not enough binding liquid or not packed hard enough. Spritz a little more and press firmly.
Mix swells and fizzes out of the mold: Too much liquid set off the citric acid. Add liquid a spritz at a time and stop at damp sand.
White film on glasses: Hard water or too much washing soda. Use rinse aid or vinegar in the rinse slot, and try half a tab.
Dishes not clean enough: Scrape and pre-rinse heavy soil. For tough loads, use a commercial tab, which has enzymes that homemade tabs lack.
Tabs turn rock hard or clump in storage: They absorbed moisture. Store them airtight with a silica packet.