The bitter oranges have the appropriate size for this recipe in Junea diameter of 2.5 to 3 cm. The ones that are smaller than that are hard and bitter and the larger have seeds. To make sure that you have the right size, cut one open, if there are seeds inside, then it's too large, ideally you should have the largest size with no seeds yet.

Pick them from the tree and set them aside for two days so they get tender (not in a bag). Most fruits when they are cut from the tree produce enzymes so that they ripen faster and get sweeter. 

Wash them thoroughly and make a hole with something pointy (like a nail or a skewer) at the point where the stem was attached, that should be as deep as possible. Boil in a pressure cooker for about 25 minutes. Allow the pressure cooker to cool on its own and then open it. If you are using a pot boil for at least 1 - 1 1/2 hour. It is important that the bitter oranges get as tender as a ripe fig. 

Place the bitter oranges in a bowl of water that you change at least 4 times a day (but don't lose your sleep over it) for a week so that they lose their bitter taste. 

On the ninth day place the bitter oranges in a pot with water and boil for 5 minutes. Then, strain them. 

At the same time prepare the syrup with the sugar and the water. Boil for five minutes and turn off the heat.

When the bitter oranges and syrup cool a bit, add the fruit to the syrup and boil for an additional five minutes.

Three or four hours later, boil once more. You do this for the sugar to infiltrate as deep as possible.

On the tenth day you thicken the syrup. In the final stage of thickening, you add the glucose syrup.

Add the glucose syrup and boil over low heat until it thickens. You know that it is ready when a drop of it on the back of the spoon does not run when it cools.

Store in a jar and when it cools thoroughly seal it.