Don't know about YouTube video to install macOS on an external drive, but it's pretty simple:
1. Your existing HDD, in a USB enclosure. (The hard drive will be really slow, but if that's all you have ... ) You will need to purchase that USB enclosure, but that's easy to find. Connect the USB drive (with the HDD installed in that drive) to a USB port on your MBPro.
2. Boot to internet recovery (Reboot, while holding Option-Command-r), and install OS X, with the external drive as the destination for the install. The OS X files will be downloaded, then copied to the external drive. Your MBPro will then finish installing the system, and finally, you will get to the new user setup. If you have a backup of your SSD (I hope you do), you can restore from that backup. It will take some time, depending on the speed of your internet connection, and the speed of the HDD. Normally, the initial system install and setup would take around an hour. Restoring files from a backup might take a bit longer, depending on how much space is needed.
THAT process will give you an older version of OS X. Once you are running, you can update to Mojave.
And - #2 would be much quicker if you already have a USB flash drive, or even a separate partition on a hard drive with the Mojave installer as a bootable drive. With that, you can install directly to Mojave on your external drive, without needing to do an older system install from Internet Recovery