From the Airport - There is a direct bus from Incheon Airport which runs 6 times a day.
Subway - Deokso is on the Jungnang line of Seoul Subway System. The station is around 45 minutes from Yongsan station and 10 minutes from Guri.
Bus - There are lots of buses from all over the province to Deokso which run regularly.
By Car - Deosko is accessible from nearby Maseok and Guri in under 20 minutes.
Walking - It is possible to walk from Guri to Deokso. If you walk towards the Han river and turn left away from Seoul, its a soothing 2 hour walk to Deokso down the river. Make sure to watch out for Welshmen attempting to swim the River Han however.