how do i turn on clamshell mode on macbook pro

See the following from Connect a display to your Mac - Apple Support

If you use an external keyboard and mouse with your Mac notebook, you can close the built-in display after you connect your external display. You might have to press a key or click your mouse to wake the external display.

The following may also help: https://eshop.macsales.com/blog/64640-how-to-use-your-mac-laptop-in-closed-display-clamshell-mode/