Find the function (f(x))
Choose the point (x=a) where you want the instantaneous rate of change
Compute the derivative (f'(x))
Evaluate the derivative at (x=a): (f'(a))
If needed, use the limit definition:
(f'(a)=lim_{hto 0}frac{f(a+h)-f(a)}{h})
The result (f'(a)) is the instantaneous rate of change at (x=a)