Put a teaspoon of salt in the bottom of a jar. Push the lemon into the jar and squeeze to release its juices. Put a teaspoon of salt over the top of the lemon and keep repeating the process until the jar is full.
Make sure the jar is at least 3/4 full of lemon juice, then top up with cold boiled water.
After 3 days, store in the refrigerator. Lemons are ready to use after 2 weeks.