Thanks to ryanpq for the helpful answer ; I'd like to elaborate on the answer a bit.
To the totally uninitiated in Ubuntu, as I am:
You need to log into your Ubuntu server. First, I was trying to generate a csr from terminal on my PC, using ryanpq's:
openssl req -new -newkey rsa:2048 -nodes -keyout yourdomain.key -out yourdomain.csr
Indeed, doing it from my PC, I was prompted through the list of questions - country, name, etc - and I was told a csr was generated, but I couldn't find it anywhere on my PC.
So, log into your server with ssh or whatever you use, where the csr will be hosted on. I don't believe the csr generated on your PC is any use.
I used sudo -s to log in as a super user. on my server, giving me root access. I put in ryanpq's command and answered the questions that followed - name, challenge password etc.
At the end of it all, I don't think I got a confirmation that the csr had been generated, which would have been nice.
I typed : ls -a (which means show files in currently directory)
I saw one named mycompany.csr (my company being the actual name of my business) and typed:
cat mycompany.csr (which opens that file)
The csr file opens, displaying the csr password - everything between 'BEGIN CERTIFICATE REQUEST' and 'END CERTIFICATE REQUEST'