Select the gray bar representing the day to which you would like to apply a Custom Activity. A white "+" will appear. Click that icon to open the New Activity Form, and first fill out the high level fields:
Plan - Select the plan with which the activity should be associated
Then set up as many Periods as are needed for the new Activity. Each Period has only a few fields to fill out:
Distance or Duration
Click the Save button at the top to create the new Activity Template
New Custom Activities can also be saved to your Library as Activity Templates for reuse later. Just click the Save to Library button before clicking the save button.
Any Activity on a Calendar can be marked as a Goal. Simply click the Goal icon on the Activity.
While creating a Custom Activity you can search your Library for existing Activity Templates to reuse their content in your new Activity. Simply search and find the existing Activity Template and drag it into the drop zone below your new periods to copy it's content into the new Activity.