nim-lock:

Alrighty!

So to make a repeating pattern: 

1. Open a canvas and draw in the center WITHOUT touching any of the edges. 

2. [Below: Light cyan lines] Divide your canvas into four equal parts.

Make four guides framing the canvas, and two across the middle. 

For the ones in the middle, View > New Guide > Horizontal or Vertical for needed orientation > type in 50% to get the exact middle of your canvas. These are photoshop shortcuts, may be different on your programs. 

3. [Above: Yellow arrows] Make sure you have ‘Snap’ checked under View > Snap, then select each quarter of your piece > layer via cut > move diagonally to the other side.  

After you’re done moving things diagonally, you can either turn off Snap so your doodling won’t stick to the center lines, or toggle the guide visibility off (Ctrl+;). Merge the layers back together. 

4. Draw in the middle without touching the borders, color the entire thing if you plan on doing that.

5. Setting your image as a pattern After your image is done, merge all layers into one > select the entire canvas (Ctrl+A) > Edit > Define Pattern

Open a new canvas bigger than the one you made the pattern on, and paint bucket it in! (Instead of the Foreground button at the top when paint bucket is active, select Pattern then your pattern)

If you aren’t using a program that lets you define patterns, merge everything into one layer and duplicate & tile them manually. 

Cheers!