By default, the Digital Signatures toolbar contains tools for creating and signing digital signature fields. For more information, see the section on Digital Signatures.
If the toolbar is currently hidden, go to View > 
Toolbars and select it to show it.
Sign Document: Creates a signature field and automatically attempts to sign it.
Add Signature Field: Creates a signature field without trying to sign it.
Note: To sign an existing digital signature field, no special tool is necessary. Simply click on the field itself.