This package requires Unity 2019.3 or later. It does not support previous versions. Get it here.
Open Unity and either create a new project or open an existing project.
Open the Package Manager window (Window / Package Manager), and check the Show preview packages option under the Advanced... button. Once the Package list is refreshed, you should see the DOTS Editor package (identified as com.unity.dots.editor). Click on Install to add it as a project dependency.
In the Package Manager window, with DOTS Editor selected, you have access to a View documentation link that opens the package documentation for the current release.
This page contains the user manual, the Editor API reference, and the Runtime API reference.