I recently made it to the Nether in a Superflat world, on Hardcore difficulty. This is how.

• As soon as you start up the world, pick a direction and get walkin'. Sooner or later you'll come across a village. There are some downsides to only walking in a line, and we'll get to those later, but you'll have just as much luck finding villages in a straight line as any other pattern since you'll generate just as many chunks, and it's easier to track where you've been. Also I recommend you set render distance to max, since it there won't be as much loaded anyway.

• Once you find a village, the first thing you should do is look for a blacksmith. The most recognizable feature of these buildings is that their roof will have slabs on it. The loot you can find in the chests can be found here. Raid the chest and mark the building somehow (I usually break one of the furnaces since it's easy to spot and I'll need them anyway) so you don't bother checking it twice. Next go to the library and grab the books, then raid the farms. This will be your primary food source. If the village has any farmers, trade for as many emeralds as you can. Emeralds can be used to buy useful items, seen here. Also grab the black wool from the lamp posts for a bed or trading.

• Next you'll need tools. Find a house that's mostly logs (Example) and tear it down. Then craft a pick and get to work on a building with cobblestone, like the church. Now you should have all of the basics. It's time to move on to the next town.

• The next part is most of the same. Find a village, raid it, and move on. What you're looking for in the chests are iron for buckets, obsidian for the portal frame, or armor/tools. Saplings are helpful but not entirely useful since you already have plenty of wood at your disposal. Finally, the best thing you can find is a saddle. Horses make walking for hundreds if not thousands of blocks a lot easier. You have plenty of wheat and carrots to tame one with, and the blacksmiths also carry horse armor sometimes. Finding one is the hard part; they will likely spawn in the spawn chunks, but since those are always loaded passive mobs probably won't spawn outside of that area. This is a downside to walking in a straight line, as mentioned before; you're probably not going to get a horse if you're over a thousand blocks away from spawn. Unfortunately I had a horse for most of my journey, but lost it in an unfortunate lava accident while I was digging it a safe pit to stay in. Stupid things.

• Settling down. You can realistically do this whenever, but I settled in a nice spot I found in the middle of four villages once I had enough obsidian and diamonds for an enchanting table, and enough lava around to make the portal. When you build said base you're going to want to build a moat three blocks out and two blocks down because as you may have noticed by now, Slimes are a big problem. They don't do much damage, but there are a lot of them and it's easy to let your guard down. You'll also want a cobblestone generator of your choice, and if you've kept any you can have a tree farm as well. Here you can also build your portal either out of obsidian blocks you've collected or with lava from blacksmiths. To light the portal, you'll need to light some wood on fire next to it. Unlike in previous versions, the village paths are not made of gravel, and there's no other way to get it in this situation. This part can take a while, so be patient.

That's all there is to it! It takes a lot of time and a lot of walking, but it is possible. Since there are no strongholds, you can't find the Dragon but the wither is technically possible but it would take quite a long time to get properly suited. Sorry I didn't take screenshots the whole way through; however I might do this again and record it (maybe even try to kill the Wither?). I started this mostly as a way to kill time but eventually I became determined after losing to Slimes too many times.

..or at least I would have completed it.

Lava bucket in hand, I had just completed the portal to the nether when I decided, "I just finished this, so I should put the lava buckets back in the moat and get my items sorted before I go into the Nether." So I go outside to put the lava back, and a Slime comes up behind me, likely newly spawned and knocks me into my own lava moat. I manage to get out with only 2 hearts remaining, but I burn to death before I can pull out my water bucket. My items fly everywhere and the screen turns red. Literally seconds from saving myself, my objective completed but the prize yet unobtained, I sit with my head in my hands.

I hate Slimes.