1. Install Tomcat 7 package.
  2. Edit the following file: 
    /volume1/@appstore/Tomcat7/src/conf/tomcat-users.xml
  3. Add the following entries: 
    <role rolename="manager-gui"/>
<user username="admin" password="<PASSWORD_HERE>" roles="manager-gui"/>

    Note: Update the token <PASSWORD_HERE> appropriately.

  4. Access the manager-gui using the following address format: 
    http://<DISKSTATION_HOST_IP>:7070/manager/html

    Note: Update the token <DISKSTATION_HOST_IP> with the IP of your Synology Diskstation.