Jeonju is a city in South Korea. It has a nice little old town part to wonder around in.
North towards Seoul or Daejeon
Ask for the nearest bus around you to the World Cup Statium, from the old part of the city there is 416 going there. Get off at the stadium and walk along the river that is running east of it. You will see a little path going along some fields. From the stadium there will be 4 bridges in total going over it. Take the last one (at that point right in front of you you will see 2 highways going over it as well) to the left where you will see a tunnel that passes one part of the highway. Keep walking straight on that road until you hit a bigger street. On your right hand side you will see a building with a Temple like rooftop. That is your toll gate. Now you just have to walk to the right side of it. There is a staircase you can't miss it. Officers might tell you that it is illegal with a bit of luck you will get a ride before that will happen (very likely).
Alternatively you can hitch in the ramp, found it easier to reach and got a ride in 30 minutes. You must have a sing here. The spot is here. There is plenty space to stop.
East towards Daegu, Busan or Gyeongju
You can start hitchhiking on the road 26 going east. Just after crossing the bridge with the railway, keep walking straight. You will cross a first gas station. You can give a try here, but it's not the best, not much people stop there and eventually they also go back to the city after. The best is keeping going till you reach the SK gas station called "Taepyongyang Gas Station" on OpenStreetMap" (there is even a GS25 if you need some snacks). Here as usual in Korea, say hello to the employees of the gas station showing them where you are going (they will ask all the customers to pick you up, they always do, they are so nice) and then stand at the exit or even at the entrance close to the highway as there is a ramp, so even cars which were not supposed to stop at the gas station have plenty of space to stop safely.
See the hitchhiking spot on map.