Tiller works with macOS 10.9+, and requires software to work with your computer. You can download the software at gettiller.com/download.
Once you've downloaded the Tiller software, you'll find it in your Downloads folder on your computer.
Double click the file, and after a few seconds a window will appear:
Drag the "Tiller" icon into "Applications".
Then go your applications folder, and double click the Tiller file.
A dialog will appear asking you to confirm you'd like to open the application:
And you've installed Tiller!