Inside the blueprints folder take the file you downloaded and rename the folder to "1, 2, 3," etc.
The settlement menu should then detect the file.
Example:
Hello everyone!
This settlement build was inspired by Favelas in Rio de Janeiro. I did my best to capture the feeling of those slums and recreate them in Fallout 4, I hope you enjoy it!
This is my first settlement Blueprint. I was told not every item will transfer. I tested this out and everything except a few beds made it through, which should be easily replaced after importing the settlement.
Please feel free to post pictures and videos of the settlement in your game, I am interested to see what you all do with it!