When you started the game you'll spawn in a random place. You'll just do what you do in normal survival Minecraft.
You'll upgrade your armor, tools, etc... But in UHC you need to be fast too. Because you have just 100 min. to upgrade yourself.
Also, there will be a world border which will get closer until it reaches 100 blocks.
And in addition, I added a bossbar in the top so you can see how much time you have until last 100 blocks.