The user assigned to a task will receive a dashboard notification asking them to complete the task, and will display the due date of the task if applicable.
Clicking on Mark as done on the right hand side will complete the task. Alternatively, you can view the task via the My Profile page, then clicking on the Job tab and Onboarding from the drop down options.
From here you can scroll down to see all tasks assigned to you. Ticking the box next to the task name will set the task status to completed.