Putting it all together
First, open your mod in Unity. If you don't have a project, please visit Introduction.
Go to Scenes and double-click LevelScreen. You may be zoomed in, so you might have to scroll out.
Go to Canvas/Inventory and duplicate any cell button. Move it wherever and change the name, and texture in Image component. In Editor Button component, replace Tool with your cell's tool, and set a keybind if you want to.
Open Placement Manager, expand Buttons if it is collapsed, increase Size by 1 and drag your newly created button to it to the bottom of it, replacing duplicate RECT.
In Scripts object do the same, expand Cell Prefabs it it is collapsed, increase Size by 1 and drag your prefab you created earlier to it, BUT...
Unlike Placement Manager, in Grid Manager you will need to exactly mimic the pattern found in Scripts/Enums/CellType.cs, otherwise it will place another cell instead of your cell.
And we are... done! Wow this sure was a journey. Click Play button at the top of the window to test your creation!
If you hit Play in the LevelScreen it won't run the game properly, you will need to either open the pause menu and go to title screen and create a new level, or change scene to MainMenu beforehand and start from there.