Assuming you're using minimum automation, this is a fairly straightforward task.
The DocuSign managed package installs an object called "DocuSign Status". This object has a lookup to the opportunity that you will take advantage of.
Create a process builder for this object that fires when it's created and each time it is edited to meet criteria.
This PB should use a time based action to fire a pre-made email alert to the specified user if the "status" field is not "completed" by the reminder date.
Here are some documents that might help you:
I just completed a POC for a completely automated DocuSign send using their Rest API in Salesforce. You're fairly limited in what you can do using the method above. For instance, your reminder email will not be able to contain a link to the document to be signed meaning if the end user for some reason loses the original email they will have to request a new document from you.
I'd recommend a seniored SF developer or a consulting service for a more advanced integration.