In order to assign them to a place on the checklist, you should select the document you would like to assign and click the Assign to button from the right tab. You will see the list of documents from your checklist. Select the task and your document will be assigned to it. Once the document has been assigned it will disappear from the Unsorted document list.
If the required task is missing in the dropdown list, select the Create new option, choose the required checklist, and enter the task name. Once you click Save, the document will disappear from the Unsorted document list and will be assigned to the specified task.