An info method essentially consists of a function which given an Emms track returns the appropriate info for that track.
We can for example look at the predefined method for retrieving information about audio tracks in the Ogg format.
The function emms-info-ogginfo provided by emms-info-ogginfo.el accepts an Emms track as a single argument and returns the appropriate information string.
We then register our info function with Emms by adding it to the emms-info-functions list. The function will then be called at the right time to provide track info.
(add-to-list 'emms-info-functions 'emms-info-ogginfo)