Put the fruits in with their juice and add sugar to taste - the more sugar you put in, the more dense the jam will become. Add the vanilla and stir well while it boils.
Finally, add the lemon juice and boil for another 5 minutes. Pour the finished melon jam into jars while still hot, close and flip over with the cap down.
Optionally, you can sterilize them for 30 minutes after they come to a boil.