Go to the "Cumulatives" section on your navigation bar or Menu > Cumulatives (if you are part of a Network) . Then, click on "add a cumulative" and select a station. Give it a name, select the type of cumulative and the right settings. Then, click "add this cumulative".
You will find all your cumulatives in the "My cumulatives" tab. Modify them by clicking on the three vertical dots near the cumulative's name.