How to make a seamless pattern

It seems that a lot of people don't know how to make a seamless pattern.



This is a simple way of making one using the center and 4 corners principle. Anyone should be able to make a decent pattern in a program that supports layers.
A related tutorial Bigger Backgrounds (from smaller graphics) will allow you to make a seamless tile without a layer-supported program such as MS Paint although you have to be more careful since you only get one chance to position things correctly after pasting.

Let me know if you find it useful or if theres something you don't understand.