How to open workflows
When assigned a workflow task, you can access it by selecting your avatar in the top right corner of your Canto page, and select Workflows. This will give you a menu of all of your workflows. You can also access workflows from the Message Board.
To open a workflow task, select the eye icon next to the task.
How to edit, approve, reject, comment
This will give you options on what to do with the task, editing it, including Approving it as complete, Rejecting it, and commenting on the task. 
Approving the workflow will allow you to insert an optional comment on the task before designating it as complete.