Determine your variables. Define your required discount rate (required rate of return in order to accept the project) and the length of the project or asset ownership (in years).
Determine the cost of the investment. This can be one initial cash outlay or multiple cash outlays. Sum for a total cost of investment.
Project the income or annual cash flows for every year of the project investment.
Go to the NPV calculator provided by Investopedia or use your own financial calculator. Enter the variables as defined above and click calculate for a base-case NPV. Adjust income up and/or initial costs down for a best-case scenario. Adjust income down and/or initial costs up for a worst-case scenario. In a financial calculator the cash flow payments are PMT, n is the number of years and i is the discount rate.