YOU CAN TRY THIS RECIPE WHICH I FOUND FOR YOU
Soak the Esrog for 1 week in water, changing the water daily.
Slice unpeeled Esrog very thinly, and remove seeds.
Boil in water, change water, and repeat twice more.
4 cups quince apples peeled and cut up
1 lemon thinly sliced
2 lbs sugar
water just enough to cover fruit
optionally add 5 or 6 whole cloves
Cook for about 2 hours until it becomes a clear syrup.
Store in a covered jar for a few weeks.
As an added bonus, you can let it "jellify" (or age) until Tu B'shvat, where it becomes an unusual fruit marmalade to celebrate with.