Peel and cut the fruit in small cubes.
In a large pot/saucepan, place the fruit and the sugar (you need to weigh the same weight of the fruit in sugar).
Cook over medium heat for 30-40 minutes, ideally until the temperature reaches 104-105°C / 219-221°F. At this temperature, you'll reach the thread stage in sugar. If you don't own a sugar thermometer, you can lift up a bit of the liquid in the jam and let it drop slowly back into the jam. It should form a continuous thread as it falls.
Remove the jam from the heat and blend half of it while it's still hot. You can use a regular blender or use an immersion blender directly into the pan.
Add the lemon juice and mix.
Keep the jam refrigerated in an air-tight container for up to 6 months.
Once cold, see if you like the texture, especially if you didn't use a sugar thermometer before. If it's too dense you can add a bit of water until you like the texture.