Simple printer test cubes.

When leveling the bed of my printers I:

1. verify spacing between the nozzle and the bed at all 4 corners using a piece of paper.
   The nozzle should be close enough to the bed that I can pull the paper and feel a bit of resistance but the paper buckles when pushed under the nozzle.

2. Run a print job with a 10x10x3mm 'cube' on each of the corners of the print bed
   Run this multiple times - adjusting each corner based on how the first layer of each corner is laid down. (hint its ok to stop the print job and restart it after you inspect the first layers)

3. run a print job with a 10x10x10mm cube in the center of the bed. If steps 1 and 2 went well you should get a perfect first layer (unless your bed is warped). This cube will let you evaluate other print settings like print speed, nozzle temp and fan settings.

After you get a 'perfect' cube move on to more advanced test tools like temp towers and overhang tests to fine tune temps, speeds, fans if you want to really dial things in.