Wash the fluffs off of the quinces. It's easier if you use a clean sponge.
Carefully with a sharp knife, dice the quinces. You don't need to peel them or cut out the cores, it contains more pectin, which will act as a gelling agent.
Add the diced quinces to a large pot and pour just enough water to cover them.
Cook them until the pieces are soft.
Blend them with a hand blender.
Press the fruit pulp through the food mill. Be very careful, because it's gonna be hot!
Add the sugar to the strained quince paste, mix it well.
Add the lemon juice to the mixture.
Put it back on the stove and cook it on low heat for 1.5-2 hours. Stir it frequently.
Be careful as the paste starts to thicken it tends to bubble which can burn you, but at this stage, the paste burns easier too, so stir it frequently!
When you can see the bottom of the pot while stirring, you can carefully pour the paste to a larger non-stick baking pan.
Leave the paste in the pan for 1-2 days, then turn it out onto a large piece of baking paper. To avoid getting any dust or any other thing on your paste, you can add a piece of baking paper on the top too. Let it dry, turn it every other day. It depends on your taste (or patience) when you decide it's ready. Personally I prefer it on the drier side, so I left mine for 2-3 weeks.