Step 1: First off you need to download a map.
Step 2: Now that you have the file, it is most likely in .rar or .zip format. Unzip it with your favorite programme. If you don't have one, download and install WinRAR.
Step 3: Rename the file that you just extracted (it is most likely named World) according to which slot you want it installed to. For Example: If you want it as World 5, rename it World5.
Step 4: Now copy or cut your newly acquirred World5 folder.
Step 5: Go to Start button. Search %appdata%
Step 6: Find and enter the "Roaming" folder. Once inside that folder, find and enter .minecraft
Step 7: In the .minecraft folder, find "saves". Paste your "World5" folder into saves.
Step 8: You're good to go. Play on your new world. Have Fun!
Any issues? Post below!