Here is a documentary you should watch before you plan to go to Kanchanjunga Base Camp (ENGLISH)

Trek with @Osan_boundless_journey
Music: https://www.bensound.com/

Permits:
It is compulsory as well as recommended to trek to KBC with a Guide.(1 guide per group)
I recommend Oshan   / oshan.rai.58  
1. Kanchenjunga Restricted Area Permit (RAP) USD 10 per person, per week.
(Obtained from Katmandu. Your Guide can Help). Also you do not need a Permit Upto Gunsa so 2 weeks should be good but its Recommended to get it for 3 weeks.
This permit must be processed through a valid local travel agency registered in Nepal. You will need a minimum of two trekkers in a group to obtain the permit.
Some people get a second permit without a trekker(Ghost permit) to get past this restriction.
You will need:
Valid passport with at least six months remaining validity and a valid Nepal visa extending past the expiry date of your RAP.

You need to supply details of
Application
Itinerary
Insurance details for guide
Online submission id (as provided by agency)
Details of dollars
E-copy of passport-sized photo

2. Kanchenjunga Conservation Area Project Entry Permit (KCAP) Nrs 2,000 per person. There is no time limitation for the permit and can be obtained at Gunsa.

Guides need to have them covered by an insurance policy during the trek period.
Tips: Only NTC sim cards function upto Gunsa. There is Electricity upto gunsa but no Internet.

Iternary : North Base Camp first to South

1. Kathmandu to Dharan (Bus, Taxi etc)
2. Dharan to ILAM to Taplejung (Bus, Taxi etc) Recommended to start early in the Day
Sightseeing: ILAM, Taplejung, Patibhara Temple.
3. Taplejung to Sekathum ( 4 wheel drive Cab) Recommended to start early in the Day or
2 days trek to Sekathum
4. Sekathum to Amjilosa to Gyabla
5. Gyabla to Phale
6. Phale to Gunsa
7. Gunsa to Kambachen ( Difficult Route- Landslides)
8. Kambachen to Lonak (Very Difficult Route- Landslides)
9. Lonak to Pangpema (High Altitude)
10. Pangpema to Lonak to Kambachen
11. Kambachen to Janu Base Camp Return to Kambachen gunsa.
12. Gunsa to Sele le via Sele pass ( Stay is recommended)
13. Sele le to Tseram via Mirgin la and sinonla ( Exxpect ice and Bad Weather)
14. Tseram to Ramche
16. Ramche to Oktang ( Base camp)
17. Oktang to Ramche to torongten
18. Torongten to Ranipul( Close to Sekathum ) Ranipul to Taplejung or
19. Torongten to Lasya bhanjang to upper Yamfudin (recommended ) but beware of Leeches
20. Upper Yamfudin to Lower yamfudin to ILAM via Kebang( Taxi)