Recently I run into requirement to develop custom integration electronic signature SaaS solution within Salesforce platform (Force.com). This was due to problems with installation of DocuSign package from AppExchange.
This article describes main concepts to do that.
On the Salesforce side – I will use a custom object called Document that will represent any DocuSignable object (contract / offer / purchase order) in the environment. Document object will have special fields to store: EnvelopeID – string with Unique envelope ID granted by DocuSign and envelope Status that will contain status of the DocuSign signature process.
Salesforce will connect with DocuSign via REST API.
DocuSign will use standard functionality called: Connect. It will send status updates to Salesforce public (to avoid logging into Salesforce with OAuth) web service.