To require approval through a documents part:
  1. Navigate to Site Explorer>Parts
  2. Locate the documents part in question and click the Pencil icon to edit it
  3. Under General, mark the box for Require approval
  4. Click Save
To require approval for files uploaded to Files:
Note: this can only be set on a per upload basis.
  1. Navigate to Site Explorer>Files
  2. Click Add Files
  3. Under File Options, unmark the box for Approve all Files
  4. Add files as needed, then click Next and proceed with upload
Note: notifications are not available when a file is ready for approval