Cheaper but more transits.
Go to Gangnam Station, Line 2, Exit 6 or 7. Walked pass by the Dunkin Donuts and cross the road. The bus stops are in the middle of the busy road. Wait for 5002 bus to Everland.
Go to Jamsil Station, Line 2, Exit 6. Wait for 5800 bus.
The bus fare is 2,000 won but if you transit from subway then the fare will be the remaining balance. For example, from Myeongdong to Gangnam costs 1,150 won and once you board the bus the remaining is 850won. ^^
Duration: about 2 hours (subway + bus ride + shuttle bus to Everland's main gate)
Easy but slightly expensive.
Non-stop tour bus (round trip).
Venue: Seoul City Hall, main gate of Korea Press center.
Time: Seoul to Everland - 9.30am / Everland to Seoul - 6.00pm
Fare: 10,000 won
Duration: ~ about 50 minutes
Reservation: Please call Daesung Tour - 031-919-1244
Note: You should stay till 9.00pm to view the fireworks (basically spend one whole day), so it's impossible if you use the non-stop tour bus.
FYI, there's also a bus to Everland from Sadang station (Line 4/ Line 2) - Bus number1500-2 and you will be tempted to ride this bus if you stay around Line 4 or 2.
My advice, DO NOT RIDE bus number 1500-2. It took me about 1 hour 20 minutes to reach Everland!!! I was so pissed off cause I thought that instead of going from Gangnam Station it would be faster, but it wasn't. If you take bus number 5002 from Gangnam Station, it will take only about 40 minutes.