Grate the peel of the citrons with a fine grater. 

Cut them in half horizontally and score their peel vertically in 4-6 places. 

Remove the pieces of peel carefully and cut each one into thin strips of about the thickness of a finger. 

Boil them in plenty of water for 10 minutes and discard the first water. 

Place them in tepid water and boil once more for an additional 10 minutes. 

Strain the peels and place them in a large bowl with plenty of cold water for 24 hours changing the water often until the are no longer bitter.

Strain them and let set them aside to drain. 

Place the water and sugar in a pot to boil for about 15 minutes. 

Add the peels and boil over strong heat for 5 minutes, allow it to let out its liquids and boil again on the next day until the syrup thickens, skimming off the foam. 

Just a little while before you remove it from heat, add the glucose syrup and the lemon juice, shaking the pot (not stirring with a spoon because that will squash the sweet). 

Your spoon sweet is done and has thickened when if you take a little of the syrup with a spoon and you allow it to run, the drops on a small plate stay tight and round and not expand.