Our journey to Sikkim
The permit is called « Inner Line Permit » I managed to access it from the Rangpo checkpoint itself, it is free and quick to get from there, especially if there's no-one else waiting. You can only remain in the region for a limited time and will need to provide your plan, state how many days you are staying, as well as hand over photos and copies of passport. I didn't have my passport copies but there is a photocopy shop next to the office. You are then given a stamp on your passport and a paper stating your rights of stay in the region. Always keep this carefully with you, as you might be asked to show it in different places and hand it back once you return to the checkpoint.
Military – You are likely to find numerous checkpoints, as your Indian VISA is not covering this part of the country. You will then need a permit to access the region and you will be asked to show it at different checkpoints. Borders are covered with military camp, Black Cat Army in order to protect India from Chinese invasion.