Login to your Salesforce org where the package is installed.
Check for Lookup between Secure EmailMessage and source object. If there is no lookup, create a lookup relationship between them.
Create a Button named as Send Secure Email on Contact(Source object).
Action Type:- Lightning Component
Height:- 450px
Lightning Component: cm_datatransfer:sendSecureEmailAura
Label: Send Secure Email
...
After creating the Email Service, add the created Email address as forwarding rule to Org-Wide Email Address so that the incoming mail will be redirected to Salesforce.
Congratulations! You have successfully installed the Application.