Either:
> Download the different parts of the audio tour directly onto a phone on which you can play mp3s
> Download the different parts of the audio tour onto your computer, and then transfer the mp3s from your computer onto a portable mp3 player or an iPod, or onto a mobile phone on which you can play the mp3s.

The tour starts from any of the three different locations:
For Tate Britain, stand in the lobby just beside the stand where audio guides are for sale.
For the boat journey simply press play once you have boarded the boat and sat down.
For Tate Modern stand at the entrance to the Tate Turbine Hall at the top of the ramp leading down into the hall.
For the Tate Britain and the Tate Modern, you will receive directions as part of the audio tour as to where to go inside the building.