List the values of the needed variables. Let the wages paid for labor be "w." Call the amount of labor used "L" (this can be a function of the units produced and labor needed as well as a specific value). Label the cost of using capital "r." Define the amount of fixed capital as "K."
Solve the relationship between the units produced and labor for L if L is not known. Often, you do not directly know the labor needed, but do know how the output of production is related to labor. In this case, you have a function x(L,K). Solve this function for L. For example, if x(L,K) = sqrt(L*K), where "sqrt" is the square-root function, solve the function for L by squaring both sides and dividing by K. The solution here is L = x^2/K.
Put L in numerical form or in the form of a function with only one variable (namely x). If L is a function of x and K, use the known value of K to remove that variable from the equation. For example, if K = 100 and L = x^2/K, write L = x^2/100.
Multiply w by L. For example if w = 20 and L = x^2/100, L*w = x^2/5. Call this result "HS."
Multiply r and K. For example, if r = 1 and K = 100, rK =100. Call this result "RHS."
Add LHS to RHS to yield the STC. In our example LHS + RHS = x^2/5 + 100. This function represents the STC of producing x units.