Here is a method, I found on another forum. It was spring and the tree was fully leafed out.

I sliced each side down through the cambium layer and into the wood. I then put the tip of a round toothpick and jambed it into the flap to hold it open. I dusted it with hormone, then below the cuts I twisted a bonsai wire tight around the trunk.

The wire also caused the trunk to swell and gain diameter size.

I then soaked a peat pellet and sliced it down the side, wrapped it around the wound and covered it with celophane, zip tied top and bottom to hold in moisture, and then wrapped in aluminum foil to prevent it from scorching in the sun.

Later that Fall, around Sept. I unwrapped it, cut it off and repotted it and now has a good root system holding it up.
This is a pretty elaborate method, but is most likely to be successful.