I just checked Naver Maps. The closest is in Ilsan. It's roughly one hour away.

You need to go to Door 8 and take bus number 3300 at Platform 8A. This bus is a "red bus," not a airport limo bus. The red bus is an express bus that's white, white, well red numbers and a roof. Sometimes, they can also be a dark orange color (these came first haha).

Once on bus 3300, it's like 2,250 KRW. Pay with cash or T-Money. Exact price, I can't ever remember because I just pay with my T-money card. T-Money is usually 100-200W cheaper.

I've never been to this Costco, so I'm going to do this based on Street View.

You take it three stops to Baek Seok Dong stop (pronounced Beck Sok Dawng). It's a bus lane in the middle of the street. On your left you will see a nine-story or so apartment complex.

When you get off the bus, you need to be on the side of the street walking against traffic (it's coming towards you). Or, in greater detail - step left off the bus. When you get to the redlight at the bus stop island, go to your left towards the apartment complex. Then turn right when you get out of the cross walk.

You'll walk one block and see a giant store Home Plus diagonal right and Hotel California directly ahead. Turn left at this intersection (stay on your side of the block, don't cross). Costco is one or two (small) blocks ahead on your left.

It's not hard, I'm just trying to be incredibly detailed as A) you have no idea where you're going and B) probably won't have a phone. I would try and use other stores you see along the way, but the small shops change all the time. That Home Plus is still guaranteed to be there.

That being said, if you get lost off the bus, just say KO SEUH TEUH KO. They drag it out exactly like you think they would. Co Suh Tuh Co. As it's their hood, someone should know.