The steps are documented in the Help Center. Just do a search on mashup.
The high level summary is:
1. Create your URL mashup in Administrator or Application User Mgmt WC
2. Then u need to place the URL mashup on the UI - go the screen where it's needed. Do a Personal Adaptation if it's for your use only OR Adaptation for all users, and activate / place that mashup in that UI screen.
All of the above requires u to be Admin access.
Good luck,
Tim