Drive 3-4 stakes into the ground a foot or two away from the tree and outside of the tree's root ball. Aim to create a 45-degree angle towards the tree's trunk with the stakes.
Next, manipulate the trunk into the right position, which is easier to do when the soil is damp. Your goal is to move the root ball enough that it grows in the right manner.
Then, secure the tree in that position by tying a rope around the trunk and looping it back around the stakes. Use straps designed for this process to minimize damage to the bark.
You'll need to come back to the tree every few weeks to ensure it has the proper hold. It can take several months for larger trees to become stable enough to grow into place.