The main ingredients used for biscuit manufacture are wheat flour, fat and sugar. Water, which is present in flour at about 14% by weight of sample (that is 14% of the flour is water), is also required at the dough mixing stage, along with other ingredients such as sugar syrups, but these are largely removed during the baking process. Water does, however, play an important role in biscuit making.
Most biscuits are made with soft or weak flour. Strong flours, which are ideal for yeast raised products such as bread, tend to bind up the mixture and not allow the oven spread which is desirable in most biscuit types. Check the labels on flour when you are buying it. Ensure that you get the type best suited to what you want to make.
Because many biscuits are made with minimum liquid, a soft-grained sugar should be used (castor or brown). Where little or no moistening agent is used (note that eggs are considered liquids), pulverised sugar which is coarser than icing sugar is recommended. Sugar needs to dissolve in a biscuit mixture to avoid dark specks of caramelised sugar appearing on the surface during baking.
If syrup or treacle is used, it should be warmed or cooled down to match the consistency of the other ingredients to ensure that it will mix in easily. Be careful to weigh/measure aerating materials such as baking powder or bicarbonate of soda accurately.