Weigh out the mass of the solvent (the substance in greatest amount) and the solute before mixing the solution. If you have a pre-mixed solution, record the masses in grams for each from the label.
Calculate the number of moles of solute by multiplying the amount in grams by the molar mass of the substance. Use a periodic table to find the molar mass.
Divide the moles of solute by the amounts of solvent in kg. This gives you the molality of the solution in moles solute/kg solvent.
Use a handbook in chemistry or a sheet over printed values to find the freezing point constant of the solvent. Multiply this constant by the molality from step 3 to find the freezing point depression. This tells you how much the freezing point has been lowered from the pure solvent.
Look up the value of the freezing point of the pure solvent, e.g. 0 degrees Celsius for water. The new freezing point will be this value minus the depression value from step 4.