1. Download the last version of piwik from the official site.
2. Upload the files to a folder in your server.
3. Go in the web browser to the folder where you uploaded piwik. Follow the instructions to install piwik. To get it working properly I needed to install piwik in the same server where my web page was, but I’ve read it possible to track sites from a different server.
4. Is recommended to use a different database for piwik, for security reasons, but you can use a database with other tables if you need.
5. Install piwik plugin for wordpress. In the ‘Piwik settings’ tab Insert the url where piwik is installed, and the auth token that you can find in your piwik installation.
6. Go to ‘tracking’ tab and only check the box (It worked in my case)