How to Reach Sikkim by Air

Sikkim Helicopter Service
Gangtok to Bagdora - 11:00 AM
Bagdogra to Gangtok - 2:30 PM
Flight Duration: 35 minutes
Cost: INR 3500 per person
How to Reach Sikkim by Train
The nearest major railway station is New Jalpaiguri (NJP) which is about 6-7kms from Siliguri and 130 km from Gangtok. Very few trains pass directly through Siliguri, which is approximately 114 km from Gangtok. Onward journeys from Siliguri or New Jalpaiguri railway station are possible by road.How to Reach Sikkim by Road

Siliguri to West Sikkim
Siliguri to DarjeelingRight outside the Siliguri Railway Station, buses and shared jeeps are available for West Sikkim. The best route considered is usually through Darjeeling (journey time - 3 hours).
Darjeeling to Jorethang
The jeep stand is located in front of the Old Super Market, almost a kilometer away from Mall Road. The journey takes approx. 1.5 hours that is filled with beautiful views of the hilly terrain. Frequent buses also pass from the jeep stand that can drop travelers at Jorethang for a lower price.
Jorethang to Peling-Yaksom
Jeep stands in Jorethang also have shared jeeps for the onward travel to Peling and Yaksom and Gangtok.
Siliguri to South Sikkim
Shared jeeps and public buses are available right outside the Siliguri Railway Station for the journey to Namchi in South Sikkim (journey time - 1 hour)Siliguri to East and North Sikkim
Shared jeeps are available from Siliguri to Gangtok at frequent intervals till sundown. Buses and jeeps are occasionally available after sunset too, but it is recommended not to travel post-sunset due to the hilly terrain and steep valley.
Traveling between North and East Sikkim
The northern and the eastern part of Sikkim usually don't provide any independent travel opportunities but if you are lucky enough, you might get a shared ride/jeep to your destination. However, the best way to travel is with one of the group tours offered by numerous travel agents, on a sharing basis.
Sikkim Entry Permits
Foreigners (except citizens of Bhutan) are required to have a valid Protected Area Permit (PAP) or Restricted Area Permit (RAP) to enter Sikkim. Individual tourists are only allowed to visit Gangtok, Rumtek Monastery, Phodong, Pemaytangtse Monastery, Kechoperi, Tashiding, Yuksum, Mangan, Singhik. For entry into other areas, foreign tourists are required to travel in groups with a travel agency. The permits are issued for a 15 days period which can be extended to a maximum of 30 days in two splits of 15 days each. In most cases, the validity is not extended beyond that period.Indians are required to get a permit issued to visit North Sikkim - beyond Singhik towards Gurudongmar Lake, Yumthang Valley, Lachung, Zero Point, etc. and parts of East Sikkim - Chhangu Lake, Nathu La and Baba Temple. No permit is required to visit places like Namchi, Pelling, Yuksom, Ravangala, etc.

Source
There is no fee charged for the permits. You can apply on the spot at the Melli Checkpoint and Rangpo (permits issued between 8:00 AM to 7:30 PM). Although if you are not in time for this, you can apply for a permit in advance at the Sikkim Tourist booth at the Bagdogra airport itself (open 10:00 AM to 4:00 PM). You can also apply at Sikkim House in Kolkata, Sikkim Tourist Office in Siliguri, or at Sikkim House in Delhi.
There is also a slightly complicated two-step process in Darjeeling-
Step 1- Applying at the Foreigners' Registration Office (FRO)
Step 2- Applying and obtaining the permit from the District Commissioner's Office (DCO) across town.
Other than that, you can also ask for a 15 days Sikkim permit at Indian embassies abroad when applying for your visa.
Special Permits
Special Trekking permits are required for high altitude trekking organised by trekking agents (in all cases, you must be accompanied by a guide) for a trekking tour in Yukson or Utterey. The application for such permits is made in Gangtok, and it takes at least a day for the documents to be delivered to the capital and back.
For further treks up the Lachen and Lachung valleys (up to the Tsopta and Yumthang) in North Sikkim, foreigners need Restricted Area Permits.
Indians need a police permit to travel north of Singhik but can venture up the Thanggu valley to Gurugongmar Lake, Zero Point and Yumthang. Permits can be procured in Gangtok through registered agencies.
Documents needed for the Permit
- Passport-sized photographs (2 or 3). Usually, it is advisable to carry a few extra copies in case you need some at the hotel check-ins also.
- Photocopy of the passport and visa (plus the original copies too)