Rub quinces with kitchen towel to remove fuzz. Remove cores and cut into pieces without peeling. Place in saucepan with 0.5 liters water, bring to boil, then simmer on low heat for 50 minutes until pieces fall apart.
Pass the cooked mixture through a sieve to remove the peel, then weigh the resulting quince puree.
Transfer the puree to a large saucepan, add the same weight of sugar, bring to boil over medium heat, stirring constantly with a wooden spoon (mixture burns easily). Reduce to low heat and simmer for at least 1 hour, stirring frequently, until the mixture is quite thick and has darkened from yellow to reddish-brown.
Remove from heat and season to taste with lemon juice to add a slightly sour touch.
Line a flat rectangular container with cling film, pour in the quince paste, smooth it out, let cool completely, then cover and refrigerate overnight to set.