Choose small and unripe watermelons.
Prepare a 3-litre glass jar.

Arrange two garlic cloves and some celery stalks at the bottom of the jar.

Slice the watermelon into rounds and arrange them in the jar. Add a celery stalk in between slices and some celery stalks and two minced garlic cloves on top.

Pour boiling water over the watermelon and let it rest for 15 minutes.

After, drain all the water from the jar into a pan; add one tablespoon of salt and two tablespoons of sugar. Sprinkle one teaspoon of citric acid over the sliced watermelon.

Place the pan with the drained water on the stove. When the water starts to boil, remove from the stove and pour into the jar over the sliced watermelon, sealing the jar with a metal lid.

Shake the jar to dissolve the citric acid. Cover the jar and let it cool completely.