The standard enthalpy of formation for a compound expresses the change in enthalpy (it can be positive or negative) when 1 mole of that compound is formed from the most stable form of its elements (at 1 atm and 273.15 K).
Standard enthalpies of formation are thus calculated for reactions that produce 1 mole of a specific compound. For example, you could write the balanced chemical equation for your synthesis reaction like this
This reaction will also have a change in enthalpy,
To get the reaction for which
That's how you can determine the reaction for which the change in enthalpy is equal to the standard enthalpy of formation for any compound.
Here's another example. Assume you want the reaction that produces
Notice that you produce 8 moles of iron (II) sulfide, but you only need to produce 1 mole
This reaction has