Remove the seeds from the cherries. Cover with sugar and allow to rest overnight.
The next day, boil the sour cherries for 20 minutes. (Complete the 20 minutes in the order of 5 minutes at high heat, 5 minutes at low heat.)
Add 1/2 the lemon juice as the jam boils and the other 1/2 lemon juice right before taking away from the heat.
Test the jam by making small drops on a plate to see whether they make a perfectly round shape. If the drop does not lose its shape, it means your jam is perfectly done.