Wash the yuzu well, and divide the peel and the insides (including the membranes).
Julienne the peel. Take the pits out of the insides and chop up finely.
Freeze the peel only. Once it's frozen, it's OK.
Mix the frozen peel and the chopped insides (with the membranes).
Layer the Step 4 mixture alternately with rock sugar in a sterilized jar. Leave as is for 2 weeks at room temperature. Shake the jar gently every other day.
In 2 weeks it's done. Put 2 to 3 teaspoons in a coffee cup, add hot water and enjoy.