When you open a diagram you can edit it provided it is not locked or referenced. If a diagram is locked then you need to unlock in the diagram details form.
You can then Add, Edit or Delete symbols, and make and edit the connections between them.
Adding symbols to a diagram
Use drag and drop from the Palette
Copy from other symbols. These can be on another page or from another model.
(you can run two instances of ControlDraw and copy from one and paste in the other.)
To add notes you can select Note Creation Mode from the Diagram Mode Toolbar and then drag where you want the note.
Subject to the diagram mode, you can move symbols with the mouse or keyboard or use the Symbol Controls to set symbol tagnames and text, link pages etc.
When any symbols are selected you can press the Delete button in the Main toolbar
If one symbol is selected and the Symbol controls is showing press Escape to hide the Symbol controls then press Delete
If a Group is showing press the Delete key