I taught myself how to download maps on Terraria, and it took me ages. This guide is going to save you the many hours it took me!
Step 1: Find the map you want to download. I recommend using this website.
Step 3: Download the map you want to your computer, and open the folder you just downloaded. Open it up and copy everything inside it.
Step 3: Open you Terraria folder and go into saves. You can find the Terraria folder here:
C:\Program Files (x86)\Steam\steamapps\common\Terraria
If you don’t have the saves folder, run terraria and create a world. Now when you open the Terraria folder, there’ll be a folder marked saves! Paste everything you copied into it and run the game.
Step 4: When you go into your worlds, you should see the name of the map you just installed. Simply open that, and there you go! Map installed!