just come out of station, take a prepaid taxi to airport - pl specify airport terminal (1-D or T3) to the pre-paid booth persons. since there is no fog during april, the train should come in time and you have plenty of time to go to airport. please do not take any metro by going to ndls. there is no need of going to ndls from old delhi.
if your train is in time, you can have your tea/coffee and then start towards airport.