Viewing the built in test trees - CorgiTaco-MC/Oh-The-Trees-Youll-Grow GitHub Wiki
The library ships with built-in test trees if you'd like to check how those are made and generate!
Once the mod is installed head into a Minecraft world and type the following command on a block found within the dirt block tag.
/place feature ohthetreesyoullgrow:<tree_id> <x> <y> <z>

then run the command:

You will now have placed a tree.
In a Minecraft world type the following command:
/place template ohthetreesyoullgrow:<tree_structure_template_id> <x> <y> <z>

and pick a tree piece from the suggestions.
Run the command:

You will now have placed one of the components of the tree!
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In your game directory find the
modsfolder.
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In the mods folder find the Oh The Trees You'll Grow Jar:

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Change the
.jarfile extension to.zip
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Open the zip and navigate to:
<zip_file>/data/ohthetreesyoullgrow
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You will find all the structure and world generation configured and placed features in the
worldgenandstructuresfolders. This will allow you to see how the test trees were made!
Be sure to check this out: Making And Saving Your Tree Pieces to get started on making your own fully custom trees today!