Google Map Link
Use the above Google Map destination, to which you can navigate and reach easily whether you are driving down from Delhi, Manali, Chandigarh, Shimla etc.
Nevertheless, some nearby landmarks are -
A) If you are driving yourself, take the route for Mandi-Aut-Manali. There is Aut tunnel. You should NOT enter the tunnel, before the tunnel take right turn. After crossing the hydro project you will reach Larjee. From Larjee you should take left towards Banjar (the other route will take you to Sainj valley). Before Banjar, the road bends towards left for Tirthan which should be taken.
B) If you are taking Volvo from Delhi or Chandigarh, reach Aut and then take a bus for Banjar. There will be many private bus operators going to Banjar from here. From Banjar, take the next bus or cab (Rs.100-200 max)to Tirthan.