Run the .dmg file and follow the on-screen prompts. Performance Tips for 12.8.3 Users
Then, the installation. After downloading the .dmg file, mount it, open the installer, follow the prompts. Maybe mention drag-and-drop into Applications. Post-installation steps include launching the app, entering a license code. They might need to activate online or via offline method. If they don't have a code, they can sign up for a free trial, but note the time limit. Also, installing updates via Avid Link or the Help menu.
Before downloading, ensure your Mac meets the Pro Tools 12 System Requirements :