Click on the Download .NET Core 2.1.x SDK (at the time of writing, the version is 2.1.105) button.
Now, in the download dialog box, select Save.
Now double-click on the file from the downloaded location (typically in the Downloads folder) and follow the instructions to install.
After installation, you should be good to go. Let's open a terminal and test the installation. (Applications | Utilities | Terminal).
Now, in the terminal, type the following command:
$ dotnet --version
If everything is OK, you should see output like this: ...
Become an O’Reilly member and get unlimited access to this title plus top books and audiobooks from O’Reilly and nearly 200 top publishers, thousands of courses curated by job role, 150+ live events each month, and much more.