This is a rather simple tutorial. you will learn how to make a square (gridlike) pattern.
Step1: lets start out by making a new document using the settings below
Step2: now zoom in as far as you can go using CTRL+ (1600%). after zooming in select a backwards L shape (1x1 pixel) using the Rectangular Marquee (Press M).
Step3: now fill that in with the color black while leaving the selection selected
Step4: select the whole entire area (CTRL+A). with the entire area selected go to edit > define pattern. a screen will pop up, give you pattern a name.
Step5: now to put your pattern to use. open up a image. any image will do. i am going to use a nice piece of abstract art. with that image open create a new layer above it. select your paint bucket tool (press G). now for the fill type select pattern. and where the pattern is select the pattern that you just created. now fill that new layer you just made in.
