Install

1. Download the zip file
2. Open Blender → Edit → Preferences → Add-ons → Install...
3. Select the downloaded zip file
4. Enable it with the checkbox

Location

Sidebar (N) → Item

Usage

1. Type some CJK text into a Text object
2. (Choose a font that has CJK glyphs like Source Han Serif, as always)
3. Open the sidebar (N) and navigate to "Item" (where transformation settings are)
4. Click "To pseudo vertical"
5. When you want to edit the text, click "Back to horizontal" then make your edit normally