Delhi Airport to take taxi and reach Kashmiri gate bus station, and you will get Bus and reach haridwar .
same route to return from Haridwar to Delhi , kashmiri gate bus station and bus station to airport.
Delhi is 5hours away from Haridwar. There are volvo buses, state buses and redbus that takes you from Haridwar to Delhi. Major drop point is Akshardham temple which is half an hour away from airport. Besides from Haridwar there are trains to Delhi which you can check too.