Nothing extraordinarily special here. The supported Unity Engines are 2019.4 onward.
Be sure to install the 2D Extras Package as well to take advantage of the Rule Tile system. This is a preview package and can be installed through via the git URL method found here.
The git URL is: com.unity.2d.tilemap.extras
To use Boxy Brush you simply must open the Tile Palette panel and select one of the three Boxy Brushes from the list of custom brushes near the bottom of the Tile Palette panel.