So, I was thinking about a map I wanted to make, but without using 3rd-party software. I thought up a great idea: use LAN! All you need to do is go to the menu, click "Open to LAN", activate cheats, then open your world!
Pros:
Don't need to download NBT Edit
Keeps world in same condition as it was in previously
Cons:
Other players can join (although for me, LAN isn't working on MC)
Remember to switch back into Survival/Adventure mode when you're done