The following method is compatible with Sublime Text 2 and 3.
- Open your .tpl file in Sublime Text
- In the Sublime Text menu, navigate to Preferences → Package Control
- From the drop-down list choose Package Control: Install Package
- Type "smarty" and Sublime Text Smarty should appear in the search results. Click it to install.
- In the menu you can now navigate to View → Syntax → Smarty to enable Smarty syntax highlighting for your document.
More information about this package can be viewed here.
If you end up with the Command Palette (the drop-down part of the UI) still open after installing the package, you can press Ctrl Shift P, or on Mac Cmd Shift P, to toggle it closed again.