Installation

Open the Package Manager from Window → Package Manager.

Select My Assets.
The image below shows Unity 2022.3.
The UI may vary depending on your Unity version.
my-assets

Select the RimGui package and click Install.

Dependency Installation

If you haven't installed the Input System, install it from Window → Package Manager.
The image below shows Unity 2022.3:
unity2022.3_input-system
The image below shows Unity 6:
unity6_input-system