When designing a form (in the Design form screen) while creating a template, make sure to add a signature component and then set that component to be required to be signed by the participant by checking appropriate check box under Required in the Who can fill this out option.
By doing this, if you create a document from that template, then the signature component will be flashing in red when the participant receives the document. If the participant does not fill in the required field, then he/she cannot submit the document.