There are several guides to help you with this installation. Take a look at the webMethods Install Guide for the Portal piece, Database Guide for the new oracle.bat/sh or msql.bat/sh scripts, and the WmMonitor Doc.
From a sequence standpoint:
1 - Run the webMethods Installer to install the WmMonitor package, my webMethods Server and Portal. The portal configuration will launch at the end of the install. This will configure the portal database.
2 - Run the installation scripts for the database, this will include the normal create/migrate scripts for oracle, sqlserver etc. I would use a separate schema from the rest of the IS stuff.
3- Run the new oracle.bat/sh or msql.sh/bat scripts to create the process piece of the database (Note: Even if you are not using process models, you still have to do this step in order to use the new my webMethods Server to manage audited services).
4-Configure the IS server to use the database for auditing and the associated all the JDBC pools.
Then the most important step, get all the fixpaks and service paks available for the WmMonitor so it will actually work.
This is just a high level overview of the steps, you will need to read the docs in detail before attempting to do this. Having done all this, it does work but it can be a be tricky to setup. webMethods could have made this a lot easier to do.