How to download maps in minecraft?

You go onto a website and download the map from the listing. You'll probably go through AdFly or MediaFire and reach the download link. It would usually come in a compressed pack, so you use 7-zip or WinRaR to open and extract that. I'd go with WinRaR, use either and extract the file.

Once you have the folder extracted, There are two ways you can open your saves on Windows.
1. Press [Windows] + r (run) and type "%appdata%". Go to.minecraft>saves.
2. Launch Minecraft and go to your resource pack list. Press "Open resource pack folder" and get out of that folder into.minecraft. You'll still be able to find the saves folder.

Open the extracted folder from the map pack and check for the map folder. The actual map folder would contain files like "DIM#", "region", "data", etc. This folder containing all that is the folder you'll drag into your saves.