As far as I know there are two basic approaches:
1) You remove the IR blocking filter from infront of your camera sensor, replacing it with a clear glass filter (or it might not need replacing I can't recall). You then either place an IR blocking or regular IR filter on the end of your lens when shooting; the former allows the camera to work as normal with the filter blocking out the IR light; whilst the latter allows it work only with the IR light.
2) You remove the IR blocking filter from in front of your camera sensor and replace it with a filter that blocks all other light barring the IR. As such the camera can now only record IR light, with the bonus that you don't have to use any additional filters on the ends of the lenses when shooting.
Also note, whilst your eyes can't see IR light, if you modify a camera for IR shooting you can use the LCD on the back in liveview mode to view the light as normal.