Choose small mandarins. They aren't generally coated with wax emulsion since they're considered lower quality.
Put the whole fruits in a pot with a thick bottom and pour cold water over them. Put them on the stove on moderate heat. Leave to come to a boil and count off 2-3 min.
Pour out the water and immediately pour in cold water over them. Put on the stove to come to a boil again, pour out the water again and repeat this process 3 times.
After the final boiling, put the fruits under running cold water. If there are any cracked ones, set them aside. Take out of the water and carefully remove the flower stalk part with a sharp knife. This small opening is made so that during boiling their insides fill with syrup.
Pour the listed amount of water, sugar and some cinnamon in a pot, put on a moderate stove, while stirring until the sugar melts. Put the fruits in this syrup to boil on low heat, simmer about 1 - 1.5 hours. After the time has elapsed, remove from the heat and leave them for 12-24 hours. While they're still warm in the beginning, leave them without a lid, later place a lid or cotton towel over them.
The next day, continue boiling on a low stove until the syrup gains the desired consistency - about 45-50 min. Toward the final few min., add the citric acid dissolved in 1 tsp water.
Distribute the ready jam into dry and hot jars with a wide opening.
Source: Maria Miteva's culinary blog.
Notes: The jam has a unique taste, with a fragrant mandarin aroma, and even though it takes quite a long time to prepare, it's worth it. I boiled a bit too long the final time, so I didn't have too much syrup but the fruits themselves are filled with it and are very tender. Once you take a bite out of the fruit you'll hear a pop and then the release of mandarin aroma.