Just open the inventory menu, select the “More” tab and click on a button.
The mod itself does not add these buttons on its own, instead other mods need to depend on this mod.
Example:

(Your inventory menu might look different because of different mods.)
Download: 1.1.0
- Version: 1.1.0 (this is a SemVer)
- Works in Minetest 5.0.0 or later
- Mandatory dependency: sfinv (part of Minetest Game)
- Git project page (with readme file): http://repo.or.cz/minetest_sfinv_buttons.git
- API documentation (important for modders): http://repo.or.cz/minetest_sfinv_button ... er:/API.md
- License of everything: MIT License
- See also: Unified Inventory. Unified Inventory has its own button system. If you use Unified Inventory, you do not need to use Simple Fast Inventory Buttons.