Actually you don’t.
You have not asked a single specific question here or in your basically duplicate) thread Install Direction for Sony 10 III.
You are just:
So fearful of messing up this phone.
You cannot “mess up a phone”, if you are sticking to the written guides (Jolla’s and / or mine). For installing SailfishOS on an Xperia 10 III there is no reason to go into specialities as “enlarging the root volume”; a fact which is also mentioned there.
You just have to read thoroughly (and until the end before installing, then read thoroughly again while installing), take your time and think before acting (both, during the installation and when writing posts). You can halt at any step of the installation and ask something specific in a concise manner here.
You seem to have successfully installed SailfishOS on your Xperia XA2, a couple of 10.000 people have successfully installed SailfishOS, so why are you afraid of doing it again?
But if you are so unfocused as you seem to be when writing these posts, e.g. spelling “A2X” when supposedly “XA2” is meant, I agree you will have a bumpy ride when running though the installation process.