Trunk Piece

To begin we need to create our trunk piece. This is what will be at the bottom/base of our tree.
Now we will add red wool blocks at the bottom of our trunk, these red wool blocks are responsible for filling in any air blocks that might be under our tree when generating our tree. This helps ensure the tree looks realistic by anchoring its full trunk to the ground.
Now we will add a white wool block at the center of our trunk, this will serve as our tree's origin/anchor. This white wool block is responsible for centering the tree up correctly with the passed-in position from the placed feature's placement. ONLY ONE WHITE WOOL BLOCK IS ALLOWED
Now get a structure block and switch it to "SAVE" mode.
In the "Structure Name" put namespace:features/trees/<tree_type>/<tree_type>_trunk1.
Adjust the relative position and structure size to fully encompass the tree trunk and the wool blocks:
Go back into the Structure Block and press SAVE on the right-hand side:

Canopy Piece

Next we need to create our canopy piece. This is what will be at the top of our tree.
Now we will add red wool blocks at the bottom of our canopy, these red wool blocks are responsible for filling in any air blocks that are between our tree's canopy and tree trunk. This attaches both pieces.
Now we will add a white wool block at the center of our canopy, this will serve as our canopy's origin/anchor. This white wool block is responsible for centering the tree's canopy with the tree's trunk. ONLY ONE WHITE WOOL BLOCK IS ALLOWED
Now get a structure block and switch it to "SAVE" mode.
In the "Structure Name" put namespace:features/trees/<tree_type>/<tree_type>_canopy1.
Adjust the relative position and structure size to fully encompass the tree canopy and the wool blocks:
Go back into the Structure Block and press SAVE on the right-hand side:

Exit your singleplayer world.
Press "SinglePlayer"
Select the world with your pieces.
Press "Edit"
Press "Open World Folder".
Open the "generated" folder.
Right-click the folder with your namespace and press copy.
Paste the folder into another folder so you can save it and have easy access to it.