Place a few small plates in the freezer 24 hours prior to preparing the marmalade.
Fill the sink with cool water and add a cup of white vinegar, soaking the mandarins for about 15 minutes.
Peel mandarins and set half of the peel aside to be used later. Do not worry about removing loose pith from the mandarin as this will help set the marmalade.
Weigh the large saucepan that will be used to boil the marmalade. This measurement will be used when calculating the amount of sugar needed.
Roughly cut the mandarin segments into a few pieces, removing seeds as you go (or leaving them in if you want peace of mind setting your marmalade). Place the prepared fruit into the large saucepan.
Prepare the reserved mandarin peel. Turn the peel pith-side up on a chopping board. Using a spoon, remove the white pith from the mandarin peel. Discard the white pith. Then slice the mandarin peel thinly into julienne strips. Place the sliced peel into the saucepan with the mandarin.
Weigh the amount of fruit/peel in the saucepan (accounting for the weight of the saucepan). Then using a 1:1 ratio, add the same amount of sugar to the pan. For example. 2kg of fruit/peel, so 2kg of sugar will be added. (These few steps can be completed the night before and the saucepan can be covered and placed in the fridge for boiling/processing the next day.)
Bring the fruit, peel and sugar to boil on medium-high heat. Reduce to medium heat once boiling. Cook, stirring occasionally, until the marmalade shows signs of setting (Use the cold plate test: Place a spoon of mandarin marmalade onto the cold plate. If the marmalade wrinkles when touched the marmalade is probably set). The time for setting can vary depending on the amount of marmalade being processed.
Place the mandarin marmalade into jars and preserve using the water bath method for 15min.
Allow the jars to sit for 24 hours. After 24 hours the lids should have sealed. If not sealed, re-process using the water bath method or store unsealed jars in the fridge.