Build a composite properly. Take an equal amount from every chosen point, empty them into one clean container, mix by hand with a clean tool, then reduce the pile by quartering until you have the quantity the method needs.
Three habits ruin it. A larger scoop from the easy points weights the result toward them. The loader bucket adds whatever it carried last. A sample left open while you finish the round loses the very moisture you are testing. Keep any suspect zone out of the composite and send it as its own labelled sample.