No idea myself, but found a page on the wiki for you:
https://www.ausbonsai.com.au/wiki/index.php?title=Olive
Digging Yamadori Olives
1, Chop the trunk to a reasonable height with a pruning saw or chainsaw. Usually 30-50cm is ideal
2, Dig a trench around the tree not a lot bigger then the base and chop off any roots as you go with your spade. Olive roots are brittle and can usually be chopped with a spade or hatchet.
3, Widen the trench so that you can get your spade under the tree and chop at the taproot. When the taproot is chopped about half way, you can push the tree with your foot and the taproot will likely snap.
4, Pull the tree out of the hole and backfill the hole.
5, Lie the tree on its side and saw the taproot off. If the tree is a large one you can take it home and use a saw to cut the tree flat just under where it bulges. Don't worry if it doesn't have any roots, it will grow more.
6, When you get it home, place it in a tub or bucket of water containing some seaweed solution for one or two days then plant out into a large pot or the ground. Make sure you keep it quite wet but not waterlogged. After a couple of months it will start putting out new shoots.
7, Leave it in place for a year, regularly pinching the new growth, before styling and potting into a bonsai pot.
Tip: Make sure you cut off all leaves from yamadori olives before you plant them as they will resist pushing out new shoots if they still have leaves on them, in some cases they might die.