Sorry, but I forgot how I did it. I did practice a lot during the two years I waited for mine. My most basic tip is to set your artboard (Custom Size) to 20x12. Go to File->Document Properties.
Using a 12x20 artboard guarantees repeat placement. Or it did. Maybe that’s changed by now, but why risk it?
My second tip is to set your stroke width to .008" or .203mm. This is about the width of your kerf in many materials. While the glowforge goes down the center of the path, using a line as wide as the cut will be can help you when doing delicate designs.