Elementary reactions are subsections, or steps, of a greater overall reaction. As an overall reaction tend to be unlikely due to correct orientation and speed requirements for all particles, reactions typically occur in steps that lead to the products of the overall reaction. Between these steps, intermediate compounds may form. The overall balanced equation of a reaction is the sum of its elementary reaction parts.
The rate law of an overall reaction can be determined by its elementary steps. The coefficient of a compound in the elementary reaction would be the order of that compound. For the entire equation, put together each compound and its order and cancel out any intermediaries, just like summing for the entire elementary reaction. However, once you reach the Rate Determining Step1), disregard the rest as the rate will be limited by that elementary step as it is the slowest.
If the slowest elementary reaction is not first, you must use steady-state approximation to determine the rate law2). In College Board™ simplified terms, this means equaling two elementary step rate equations, or an equilibrium reaction if present, that share the intermediate and then rearranging for the intermediate. Afterwards, we can plug in the “intermediate” rearranged expression and simplify for our final rate law. Don't be afraid! Disregarding the scientific notation this is pure algebra.
The rate determining step of an overall reaction is the elementary step with the slowest rate.