Open Unity and select Edit > Project Settings > Player to open the PlayerSettings panel.
Select .NET 4.x Equivalent from the Scripting Runtime Version dropdown and .NET Standard 2.0 from the Api Compatibility Level dropdown in the Configuration section.
Add the .unitypackage you downloaded in the first step to Unity by using the Assets > Import Package > Custom Package menu option.
In the Import Unity Package box that pops up, you can select which things you'd like to import. By default everything will be selected. If you don't care to use the sample, you can uncheck that box.
Click the Import button to add the items to your project.
With the package added, you can now use the normal API in your scripts as you would in any other application. Please take a look at the corresponding sample (also included in each .unitypackage) which demonstrates how to use each of the services to perform simple tasks.