Name your dependencies then hit 'Create'.

Click into your new task and give it a name or any more details.

Resize, add and move more tasks directly from the timeline.

You can also create dependencies from within a task page, by clicking into the 'Blocking' and 'Blocked By' property. Return to the timeline to see how the dependencies look, visually.

In your Table view, you can also see which tasks are 'Blocked by' and which are 'Blocking' other tasks in this database.

How To Create Notion Timeline Dependencies