Planning and Setting a Budget - This was super useful. I'm not the kind of person to follow plans too well but I went ahead and made some just in case. I would recommend checking prices on airfare often and deleting your browser cookies and cache before doing so. I've been told that certain websites will track your traffic to keep the price steady on a flight you check often and deleting your cookies and cache for their sites help prevent that. Also there are sites like Kayak that give you a 'heads up' on trending pricing, so they'll let you know if prices for your flights are expected to drop or rise to help you decide when to buy, even if you don't use their site to do so. I made a list of "must do/try" and tried to make my plans by locations (for ex. I wanted to try 'Dragon's Beard Candy', that was in Dongdaemun. I also planned to see their shopping district, and it happened to be near some lovely parks and a small river that led to a famous national landmark, so I jotted that down as my Tuesday plans.) I tried to bring a certain amount of money each day and leave some at my home in case I got super drunk and lost it or got mugged or something. Youtubers had the best 'must see places' so I'd recommend that to populate your list.
I used Airbnb, not only for my accommodations, but also to book 'Experiences'. That turned out to be a great idea since I made so many friends that were doing the same thing I was doing. There were a lot of solo and duo travelers, some even spoke Korean, most were about my age, and just as adventurous. I ended up making plans with a few of them for several nights and expanding my group of friends and events.
Seoul in particular was a very connected city, so I never needed a local number or to purchase portable wifi but it was available in the airports before you get on the subway. The t-money card was worth every penny and I noticed a lot of planned cities with a solid transit system offer some variant of it.
As for general tips, I'd say don't forget to call your credit card company to let them know where you'll be so they don't block your transactions. Your passport can't be expiring within three months from your travel date (at least for South Korea, it did vary by location), so keep it up to date. If you plan on renting a car, you may have to get an International Driving Permit. You may also need a power converter for your electronic devices. Vaccinations might be something you'd be interested in looking into as well. DO NOT exchange your money at the airport (just what you'll need to get to Seoul Station, in Korea's case) as they will likely have one of the worst exchange rates. Learn a few phrases or words in the local language as most people appreciate even the slightest effort in trying to appease them. Google Maps was my best friend, install that app.
Be a 'Yes Man'. If someone invites you out to something but you had plans that can be rescheduled, say yes! Do not pass up any opportunity to make friends or go to an event that wasn't planned. My favorite moments of the entire trip were made on a whim, spur of the moment. If someone smiles at you, say hi! Laugh with strangers, buy someone a drink, ask someone on a date, just ACT.
Don't be afraid to get lost. I got lost every single day. I would go one direction, notice I was getting further from my destination and u-turn, then u-turn again If I over-corrected. I even changed or straight up abandoned my plans If I noticed something was going on I didn't know about, if it sounded fun or looked cool too. You'd be amazed at how many activities you can fit in a single day. I often ended up having time to do all my planned activities with three or four hours to spare for random things that came up.
Leave some time for you to rest. They (Koreans) do sell some wonderful hangover cures, either at restaurants via soups or corner stores via energy/vitamin drinks, but you have to wake up and have enough energy for it to be worth it. Don't overexert yourself or you'll be too sore and lethargic the next day.
I was told that eating was very communal and some restaurants wouldn't even let you come in if you were dining alone. While eating in Korea was indeed very communal (no tv's, few to no people on their phones, most came in at least duo's), I found myself eating alone a few times and not once was I turned away, or stared at because of it. There were even some districts and shops where everyone was eating alone, and on the go.
Bring some music! After a few days of wandering around Korea, I eventually decided to bring my headphones and jam out while exploring. That turned out to be a great idea as it sometimes even set the mood. Some parks' wildlife was extremely serene to listen to, while only the sound of your shoes against the gravel or grass was heard, but in the subways, my music was always preferred.
Share your experience. I used Snapchat to document and share my adventures live and I ended up reconnecting with a lot of old friends and it also, in a way, added value to my trip. My journey was much more rewarding when a friend would make a funny remark or encouraged me to 'keep the pics coming' because, for at least a moment, they were living vicariously through me.
Get some post-production content. If you took a picture that could use some retouching, do that, then post it. Make a video scrap book. Go nuts, create something more polished with the raw material you accrued. It could even be little notes you took to make a journal or blog post ;P People enjoy consuming well-planned and thought-out content just as much as live/raw content.
Don't let it be just a memory. Learn from your experiences and apply them to your daily life. If you enjoyed the food, learn how to make it at home, or visit a new restaurant that makes it near you. Did you pick up a new and healthy habit? Keep it going! Make some friendships or meaningful connections? Network your butt off! You might end up with a free stay at a faraway home, or someone with a marketable skill that they'll lend to you as a favor.
Move forward. I've traveled to a few destinations and it seems like every single one has been my favorite, 'must revisit' destination. If I had done that, I would've never gotten to this point. As hard as it is for me to say this, try your best to visit new destinations as much as possible. The world has so much to offer and you'll never discover it if you keep going to what's familiar and safe. That's not to say you should never revisit, surely there's things you missed or maybe the time of the year could completely change your experience the next time you go, but I would emphasize you going to different parts each visit.
I hope I enlightened y'all on at least one thing you may not have thought of, or perhaps even inspired you to try traveling alone. If nothing else, I really hope you take it from me when I say, JUST DO IT. Stop hesitating and go. If you need any help or have any questions I may not have covered in this guide, let me know and I'll try my best to answer it.