Making up heavy syrup, sometimes referred to as 2:1 or winter syrup, is not at all difficult. Many will know the 1 lb of sugar to 1 pt of water recipe, which in modern parlance roughly equates to 2 kg sugar in 1.25 l of water.
To make up ~12.5 litres use the following recipe.
Heat up 6.25 l of water to boiling and turn off the heat. Add 10 kg white sugar and stir until dissolved. Once dissolved the liquid will be clear and slightly straw coloured. Be careful not to boil the solution as boiling can produce a molecule called hydroxymethylfurfural which can cause ulceration of the bees gut and so is potentially quite harmful.
Once cooled sufficiently I add 5 ml of thymol solution made up to the recipe on Dave Cushmans website (always be careful when handling thymol crystals). This makes a 1 x Manley strength thymolised sugar syrup which should not suffer from black mould.
For reference, and hopefully to save your back, 12.5 l of 2:1 sugar syrup weighs about 16 kg and is equivalent to about 14-15kg of stores when in the hive.
Bridget Beattie. (August 2013). Winter Feeding. Available: http://www.bbka.org.uk/files/library/bbka_news_216_aug_2013_1374844570.pdf#page=5. Last accessed 30 September 2016.