1. Boot the current macOS install
2. Use the built-in DIsk Utility to format the USB-connected drive to APFS (give the drive a different name from the current boot drive just to keep it clear which is which)
3. Reboot in Recovery Mode https://support.apple.com/en-ca/HT204904
4. Install macOS from Recovery Mode - it will allow you to choose which drive is the target for the new install
5. Once macOS is installed on the SSD, you can boot the machine with the Option key held down and be offered a choice of which drive to boot from - choose the SSD to check that it boots properly
OR, you could create a macOS installer on a flash drive https://support.apple.com/en-ca/HT201372 but it would amount to the same thing.
Perfect. Thanks

My advice would be, make your friend open the screen himself with you explaining how to do it. That way, when the glass shatters (and I'd give it a good chance it will) or the ribbon cable gets damaged, he'll see just how little force it took and can only blame himself for it.
After spending $700 of my own money to replace one I did as a favor (which I accidentally damaged the razor-thin ribbon cable at the top just below the glue) I swore to never touch one of those screens again.
I keep winding him up when he asks how it's going, telling him that I'm busy sweeping the glass up, or that it's back together with "only a few small cracks", so in a way I've prepared him for the worst

The reality is that I've barely had time to start on it yet. I have loosened the screen (top and sides) and managed to prise it forward a little, so I'm fairly optimistic at this point. He's fully aware of the risks though and has said he'll take the blame if it breaks. One thing's for sure, I wouldn't work on one of these for a customer. I'm only doing this one because I know him well and he knows that if it breaks it won't be through lack of care.