but the gist is:
1. go to Edit > Preferences...
2. on the left change the tab to "Add-ons"
3. top right click on "Install..."
4. now select botaniq_full.zip or botaniq_lite.zip that you downloaded and wait for a short while before the data is copied to your Blender AppData folder.
5. now you can find trees & more to plant in the sidebar of the 3D view under "botaniq" tab!
Have fun planting!