Enter your PvE or PvP stats, pick your class at the top, then choose the matching combat mode. Select the monster you want to test against. In PvP mode, also enter the server multiplier. If you already have a screenshot of your detailed stats window, use Upload at the top of the page to open the import guide and choose a file.
After that, check your skill values carefully, especially if the skill hits multiple targets. The game tooltip does not show the multi-target factor, and that is one of the most common reasons people enter the wrong setup. Once the inputs match your character and target, the calculator will show the expected hit values and DPS breakdown.
Helpful extra resources: the Skills Wiki for checking skill data, and the Target Damage Multipliers page for understanding target damage boost values.