I've omitted some general details, but this should get you moving:
create a PDF document with a signature field on it (http://livedocs.adobe.com/livecycle/es/wb_designer/Making_secure_forms.81.4.html)
add the credential for signing the form to the trust store (http://www.adobe.com/go/learn_lc_adminTrustStore)
create a process that includes the Sign Signature Field operation (Signature service) (http://livedocs.adobe.com/livecycle/es/wb_help/00000980.html) as well as a document variable, with the Input property selected. This document variable is the used as the Input PDF for the Sign Sig Field operation.
Create the watched folder, and configure to invoke the process, and setting the value of the document variable to the PDF that has the signature field on it. (http://help.adobe.com/en_US/livecycle/es/admin_aac/admin/endpoints4.html)