Click and drag out your canvas so it's about 800 x 800. Make sure you can see your entire canvas in your window at 100%.
Choose the Rectangle shape. Then:
Click and drag out a rectangle, not too big. You will just see a dotted outline.
Click off - you should see a solid outline like this...
If you don't, first go to Outline>Solid color - then drag out a rectangle again.
With it still selected, click on Copy or CTRL+C...
Before you paste the rectangle, zoom in to 300%...
Then use the Paste button or CTRL+V...
Set the Selection tool to be transparent...
Then you'll see the original rectangle - move the new one over to the right until they're lined up nicely like this...
Change Color 1 to whatever color you like (not black). Select the Paint Bucket tool and click inside one of the rectangles...
Change Color 1 to another color and fill in the other rectangle...
When moving into place, use arrow keys to slide it over...
Keep going until you've reached the right side...
Move what you pasted so it staggers a bit to the left/ right...
Now scroll down and paste everything down: CTRL+A, CTRL+C, scroll down and CTRL+V.
Repeat 2-3x until you reach the bottom.
Go to 100% and you should see something like this...
Use the Select tool - draw what you want to keep so you have no empty white spaces...
Then click on the Crop tool...
You should have something like this...