There usually (always?) is something like "Restore defaults" in the BIOS/UEFI
configuration. Search for it.
Trisquel's documentation suggests "the USB installer provided by
pendrivelinux.com" to create the live USB:
https://trisquel.info/en/wiki/how-create-liveusb
I wrote a manual to configure the BIOS/UEFI to boot the live system... but it
is in French (do you read French?):
https://trisquel.info/fr/wiki/d%C3%A9marrer-un-syst%C3%A8me-live-installable
Notice that it is practically impossible to provide step by step instructions
to configure the BIOS/UEFI because there are many different BIOS/UEFI
configuration interfaces.
