An adapter cable. You need a "Y" that is two RCA phono plugs going into a 3.5mm TRS (stereo) female phone jack. Tip=right, ring=left, shield=common. If you need a 1/4" female TRS jack for your headphones substitue that instead of the 3.5mm (1/8").

Ring = Right = Red (R = R = R)

Tip = Left = White. (Can't for the moment think of an aide-memoire for this one that does not involve Horlicks)

The problem is going to be that that Delta 1010LT is not specified to drive headphones. The analog outputs are line level (switchable +4dBu / -10dBV) and do not have headphone drivers. You will need some sort of headphone amplifier, even if it's only a hi-fi system that has "aux" inputs and a headphone jack. If you use aux inputs on a domestic amplifier, set the 1010 to have -10dBV outputs.

Ahhh.....
Thanks to Boswell for catching my pain addled error. I'll be better tonight. Only 8 pianos to tune today.

Im not going to use headphones for mixing - just to listen to music when people sleep :P Thanks for the info ! :)