Great question — answering as the UphillTech team:
Yes, for Har-ki-doon trek you need a local forest permit. Hiring a guide is not a mandatory thing, but recommended. The guide will take care of campsites and arranging food at a fair bargain. First things first. You can book a cab from Dehradun to Sankri villege. On the way you would pass through a place called Naitwar (Tagged Netwar on google maps). The permits to enter the "Govind Ghat wildlife Sanctuary and National Park" are supposed to be obtained here. Cab should cost you something around 3000 INR (approx 48-50 USD). There is an administrative office of Uttarakhand Tourism where you can get the permits for the trek. While, I have no idea about how much they cost you, I'd recommend...
Full answer by WedaPashi: https://outdoors.stackexchange.com/a/9660 (CC BY-SA 3.0).
Need it done for your business? UphillTech.