Put the quinces in a large pan of boiling water. Cook for about an hour or until they are tender. I poked mine with a cake tester. Try to make sure they cook evenly by rotating the quinces in the water.
Cool.
Cut the quinces in half. Remove the cores. Put the pulp in a bowl and measure it. Add an equal amount of sugar to a pan. Add the lemon juice. Stir over low heat until the sugar is dissolved. Then stir often until the mixture pulls away from the sides of the pan. Depending on the heat this can take 30 to 45 minutes.
Line a small pan with parchment paper and spray it well. Pour the quince mixture into a small baking dish. You want the mixture to be at least an inch high in the pan.
Bake at 175 degrees for 3 hours. Cool and refrigerate.
Recipe by Recipes Food and Cooking at https://recipesfoodandcooking.com/2015/09/17/membrillo-aka-quince-paste-with-manchego-cheese/