Twice as wide and deep than the roots.
1/2 cup 10-10-10 or a quart of compost. Drive in a hardwood stake.
As you backfill the hole spread the roots out and position the benchgraft so that the graft union ends up 2 inches above the soil line.
Prune off any growth originating from below the graft union. This process may last for a few years.
The buds above the graft union will open. This indicated the graft is a success.
The open buds will form new trunks. Sometimes more then one!
Remove all but the the most vigorous new growth.
When the new trunk reaches 15 to 20 inches tie it to the stake for support.
Make a screen saver with scissors by cutting a 12 by 24 in section from a roll of aluminum window screen. Bend the screen around the stake and the new tree to form a tube.
Use an office stapler to close the tube.
The window screen will protect your baby tree from rodents that otherwise, will kill it. Use the search terms tree protector tree tube, etc, to purchse one ready -to-go.
As the baby tree grows, keep removing any growth from below the graft. You may need to readjust the screen saver to keep the new tree pruned.