Smooth background help!



I want to learn how to make a smooth background. I've looked through people's code but I don't just want to steal someone's code, I want to actually learn how to do it. Thanks for any help!
If you help me, I'll give you credit whenever I make a project using a smooth backgrounds!


Continuing the discussion from How to make Smooth Backgrounds(EASY):
Just Look at this topic! It explains the code