how to boot my macbook pro from a external drive

The External Drive needs to have a Full Installation of a version of macOS this computer Qualifies to run. Example computer qualifies to run High Sierra and nothing newer - then one can install High Sierra to the external drive Once that is achieved - with the External Drive directly attached to the computer and in the Off position - startup the computer and immediately hold the OPTION key which will start the Boot Manage and choose the External Drive to boot into.

Should one whats to always boot from the External Drive again boot to the External Drive and in System Preference >> Startup Drive - it should present Internal Drive by its name OR External Drive by its name. Choose the External Drive and save.

To installed to High Sierra in this example will required Downloading the FULL VERSION of High Sierra from this link  How to get old versions of macOS and once it is downloaded run the Installer BUT make sure the Destination Drive is attached and the Destination drive is chosen for where it will be installed.