Line up your printer with your labels
Every printer shifts the page a little. Measure that shift once and every PDF from this site is moved to cancel it. Takes two minutes and a ruler.
Pick the sheet you use
Print the test page on plain paper
Choose Actual size in the print dialog.
Measure the red marks
Measure from the very edge of the paper. Units:
Or nudge it by eye
Hold the test print over an empty label sheet against a window. Move the outlines until they sit on the labels.
The adjustment is saved in this browser only. If you print from another computer or printer, measure again there.