I have really great news for you. The metro system is Seoul is ridiculously easy to use. There is english everywhere and maps galore in the crazy underground world. Plus there are endless amounts of corridors filled with food and shopping. You could literally live down there.
Some of you guys use Metros all over the world (good job!) and just want a quick version so just read below and you're good to go.
For us more nervous folks (and I am not afraid to admit - this is usually me even though I travel a ton!) first off breathe, you can do this, it's REALLY easy. Read below the quick version for a crazy detailed guide.
Also there is a video at the very end (from snapchat, sorry for the quality) that is a quick 2 minute watch if you want some extra guidance.
The quick version
The metro takes CASH only
Buy a T-money card either underground or at 7-Eleven (around $2)
Load it up with a ton of money. Don't worry about losing money you can refund whatever is left over at the end of your trip.
Each ride is a little more than $1
Transfers are free or heavily reduced
Use google maps to find your route - there is free wifi everywhere even down in the subway
Tap in (and out) to use the subway
When underground look above the glass doors it will tell you which stop is next on that side
Do not sit in the elderly seats even on an empty train
Use an exit map on your way out - the subway stations are huge and you could literally be 4 or 5 blocks from a certain exit.
THE LONG VERSION
Step 1: How to Figure out your route
The good news is your phones will work everywhere. Internet is literally on every corner unpassword protected and free. Also, there is free wifi down in the subways. (Sometimes the wifi just didn't want to work for me so I usually looked up stuff before I went down)
Use google maps to find which subway and where you are going. Usually looks like the picture to the right. There are two pieces of information that you need to read and know. First is the name at the top (circled in red). This is going to be the last stop on the train, very important. Also, you want to know what your very next stop is (circled in blue). Keep both of these in mind once you are underground.
Pro Tip: I usually take a screenshot of my train ride when I have service. Just in the slim chance that I back out of google maps by accident or it crashes or it refuses to load once I lose service in the subway.
Finding the Metro
Finding the Metro is a pretty easy feat. There are huge pillars that say METRO on them next to very big staircases. On the pillars is a nice clip art symbol of a train just incase you missed the words. They are also color coded and lite up at night.
Buying a T-money Card
Your first order of business when you go underground is getting a T-Money card. (You can also buy these at any 7-Eleven in Korea.) You can just buy tokens every time you use the metro but I would suggest the card.
First off the card gets you a discount each time you use it and it gives you free or heavily reduced transfers. Second, when you first buy the card you can just load it with a TON of money - seriously load it with way more than you need. This is because at the end of the trip you can go to a refund machine and get your money and the cost of the card back. Third, the card gets you onto the metro and bus systems for the ENTIRE country and you can also use the cash on the card for purchases at 7-Eleven.
The card costs around $2 and like I said above when you are leaving the country you can put it back in a refund machine in any subway and it will refund you the cost of the card and any remaining money on it (minus a 50 cent processing fee).
VERY IMPORTANT: the machines to buy the card and refill are cash only.
To the left are what the machines look like. The blue one all the way to the left is the machine to buy an actual T-Money card. Once you have the card you go to the next machine over (the two that look the same) to load it with money. The closest machine is the refund machine. As you can see lots of english to help you out. And again cash only for all these machines.
Loading your card with money
Loading the actual card is pretty easy. The first picture on the left (above) is the screen you start out with. Press English and then the button all the way on the right that says Reload the transit card.
The machine will tell you to place your card in the reload pocket. (the picture above on the right). You will select how much you want to load on your card and insert your cash. Wait for your card to reload (picture directly right) and once you get a complete screen take your card and head to the subway entrance.
Accessing the metro
Remember the screen shot of google maps from step 1? Before you head into the metro look up. You're going to want to make sure that your going into the right side. Some metros have just one entrance to both sides some have two entrances to the different directions. Just be aware before you pay your fare. (This is starting to sound like a Cat and the Hat book with the rhyming)
Once you figure out if you are at the right entrance just tap your T-money card and go inside and downstairs.
Figure out your line and on the train
Once you follow the signs for downstairs MOST metros have trains on either side. Just look above the doors. Each side shows you which is the next stop. For instance: The picture to the left shows I am at the Euljiro 3 stop and the next stop on this train is the Jongmo 3.
Then, once you are on the train MOST subway lines have a nice TV screen above the doors that give you the next stop in Korean and English. (picture to the right) If for some reason you get on an older train just keep an eye out at each stop there are huge signs everywhere. (Did I mention the Korean subway is super english friendly).
Extremely Important Tip: Honor, especially honor to the elderly, is a very big thing in Korea and there are special seats on the train for the elderly and disabled. Even if the train is relatively empty DO NOT sit in these seats. You will get reprimanded by younger Koreans. Also as you exit the train and you see elderly with their baskets or walkers it is not uncommon for many people to stop and help them. If no one is around embrace the culture and help them yourself.
Transfers are really easy. I am not going to go into crazy detail because you literally just follow the signs. No need to take your T-money card out either everything is connected.
Exiting the Metro
Once you arrive at your stop take a second to congratulate yourself. YOU DID IT! Use your T-money card to again tap out of the subway.
The subways stations are huge and most of the big ones tourists frequent are connected to huge underground malls. You'll want to find a map like the one above and use it to figure out which exit to come out of that is closest to your destination.
A small word on buses
The bus system is again - ridiculously - easy. The stops have clear signage and you can use your T-money card to tap in and out. Only load at the front of the bus. Only unload at the back of the bus. You have 30 minutes from when you tap out to transfer for free to a different bus or train.