As you work, use pruners to cleanly cut off exposed roots flush with root ball. Work carefully, slowly digging straight down until you reach bottom of root zone. Brush off loose topsoil to reduce weight, being careful not to nick trunk in the process.
Shape bottom of root ball by undercutting it so it tapers in. Try not to disturb it as you work; you don’t want root ball to crumble or completely detach from base of hole. Ultimately, you’ll end up with a sphere attached to a pedestal of soil.
Take square of burlap wide enough to reach from base of trunk down to bottom of hole with about 25 centimetres to spare, and long enough to wrap around root ball with an allowance for overlap. Smooth it snugly around top and sides of root ball; let “skirt” at bottom flare out.