If a planning hasn't been created yet, you'll enter an empty planning screen (fig. 1). Click on the 'Create planning' button to create one. After clicking the 'Create planning' button, a dialog (fig. 2) opens with default activities (manage in Settings > Master data). To set an initial planning, start by filling in a start- and end date of the default activity. Other activities will be calculated automatically through defaults.

Switch the toggle button to 'Manual' if you wish to specify the dates of each activity. This will enable all other date fields. When you're finished, click on 'Create' and your initial planning will be created. This planning will be applied to each object of your scenario. 

Once a planning has been created, you can further specify the planning by adding activities and milestones manually for each object or group.

Figure 1: An empty planning screen

Figure 2: Create planning dialog