These calculations are based on launches from KSC so results might be different for other bodies. I might include an option to choose which body to launch from later on.
Your payload will be the first stage to enter, so first build your payload (for example: probe/rover/sattelite). Once you have done this You define it's full and empty weight in tonnes, and it's thrust in KiloNewtons. Enter that in the calculator and press the "Add stage" button. Repeat this step for each subsequent step until you have got all your steps in the calculation.
I'd like to say thank you to the users which are listed on the list below. This keeps me motivated to do stuff like this.