Identify the total cost function, (C(q))
Identify the output quantity, (q)
Find the derivative of the total cost function with respect to quantity, (frac{dC}{dq})
Evaluate the derivative at the desired output level
Use the result as the marginal cost
If using data instead of a formula, compute (frac{Delta C}{Delta q}) for a small change in output
Keep units as cost per additional unit produced