In the SONG section of the front panel:

1. Press and hold [REC], then press [TOP].
    
2. Release the buttons, then select a Style and Voice(s). If you want to record to the MAIN/LAYER/LEFT voices make sure to set each one to ON..
    
3. Press and hold [REC], then press the Track buttons above it for the Tracks that are to be recorded. [EXTRA TRACKS] will record all the accompaniment parts. [TRACK 1] records your right-hand voice. [TRACK 2] records the left-hand Main voice..
    
4. Release the buttons, then play the keyboard to start recording.
    
5. Press [REC] to stop recording.
    
6. To hear what you have just recorded, press [TOP], then [START/STOP].
    
7. To stop the Song, press [START/STOP] again.

PSR-2100 Owners Manual, p.96