Sure, here are some tips for your wonderful journey through the northeastern part of India:

Best Time to Visit:
The best time to visit Guwahati, Shillong, and Cherrapunji is from October to April as the weather remains pleasant and rainfall minimal. For Gangtok, the best time is between October to mid-December when clear views of the Himalayan peaks are possible.

Route and Accessibility:
Direct flights to Guwahati are available from major Indian cities. From Guwahati, Shillong is about 3 hours by road and Cherrapunji another 2 hours from Shillong. Gangtok, on the other hand, can be reached by a 4-5 hour drive from Bagdogra Airport or a New Jalpaiguri (NJP) railway station in Siliguri.

Altitude and Acclimatization:
Except for Gangtok (which is approximately 5500 ft above sea level), the other places aren't too high, hence acclimatization is not much of an issue. However, it's always advised to take it slow and stay hydrated in Gangtok due to the altitude.

Accommodation and Booking:
Plan and book your accommodations in advance through popular booking platforms or via travel agents. Homestays, hotels, and guest houses are available in all these locations.

Clothing and Essentials:
Pack according to the season. If you're visiting between October to April, bring warm clothes. Always keep a good quality umbrella or raincoat handy in Cherrapunji as it is one of the wettest places on Earth. Comfortable walking shoes are a must for all terrains.

Health and Safety:
Always carry a basic first aid kit. Despite general safety in these areas, stay aware of your surroundings and keep important emergency contacts at hand.

Local Etiquette and Respect:
The Northeast is a mosaic of unique tribes and cultures. It would be highly appreciated if you respect the local traditions, dress modestly, avoid littering, and respect wildlife and natural surroundings.