Don't worry, it's the easiest thing to do in Delhi. Get out of NDLS from gate-2(Ajmeri Gate), look for the New Delhi Metro station(very much visible) and after buying a token to Chandni Chowk board the metro towards Jahangirpuri/Vishwavidyalaya. Get down at Chandni Chowk(2nd station from New Delhi) and look for the display board indicating the exit for Old Delhi/Delhi Jn Railway station, follow the directions and reach DLI.