How to boot from an external drive (liveusb or other) from Trisquel in order to install Trisquel mini

Hi everyone
First of all, english is not my native language, so you may see some errors in my writting. I apologize for that in advance.
I recently replaced win10 with Trisquel in my laptop. I'm pleased too see how it's running smoothly and does not struggle anymore to do common tasks like text processing.
I would like to try Trisquel mini in order to choose if this OS better suits my needs.
To do that i created a liveusb to boot my computer with it, and install Tmini.
However when i restart the computer with the liveusb attached, it does not boot from it and the Trisquel OS starts normally.
My question is ¿what do i need to do to boot in Trisquel from an external drive, being a pendrive in this case?
Thank you all