Determine your function. It will probably be given to you in the problem. For example, your function might be F(x) = x^3.
Choose the instant (x value) you want to find the instantaneous rate of change for. For example, your x value could be 10.
Derive the function from Step 1. For example, if your function is F(x) = x^3, then the derivative would be F'(x) = 3x^2.
Input the instant from Step 2 into the derivative function from Step 3. F'(10) = 3x10^2 = 300. 300 is the instantaneous rate of change of the function x^3 at the instant 10.