You don't have to worry at all about the permit to visit North Sikkim as the permit is made the day before you go for the North Sikkim package which is 3day 2night. The tour packages are available in many Tour and Travelling Shops in MG Marg, Gangtok. The North Sikkim packages includes (Lachung, Zero point, Lachen, Yamthang Valley, Gurudongmar Lake. The traveling agents will do your permit at the shop only but the things to carry for permit is Xerox of the Aadar or Voter Card and 1 passport size photo. I suggest you to carry at least 10 photos and 10 Xerox of the identity proof card (Do carry the originals with you). Ive visited North Sikkim it's wonderful and breathtaking š