How to create a separate /boot partition? (Ubuntu if it's relevant)

[Chris Green]

A friend (it really is a friend and not me) has an old[ish] system
running Ubuntu 18.04.

It's all on a spinning hard disk at the moment but he has bought a new
SSD and wants to run off that.  The problem is that the SSD isn't
recognised by the BIOS, it works perfectly once Ubuntu is up and
running though.

So what one needs to do is to create a /boot partition on the old
spinning hard disk which will then be able to see and mount the new
SSD which has the / partition on it.

What's the easiest way of doing this, preferably without doing a new
install?

-- 
Chris Green