Ok, I think my problem is that I'm trying to turn a standalone site into a slave site - is this possible?

YES this is possible if you have asked to copy the directories instead of using symbolic links or simply share the master directory (when deploy direction is empty).

Is there an instruction for this? I can't find it.

What do you want to do ?
Install the "Article Sharing" or create a copy of the directories.

This presented in the tutorial video that show you to create slave site from the front-end or any other video that use templates.
This is a parameters in the JMS templates "Files and folders" tabs.

What I'm trying to do is share articles (and only articles) from my master site with another site.

So I repeat that you have to install the "Article Sharing" in your master website and also in the slave site where you want to use it.

I have already told you that you can either re-install the "Article Sharing" in the slave site or use the JMS Tools to install the "Article Sharing" in the slave site.

Here it is again the full procedure summarized as described in the tutorial video.
This is a "hostgator" procedure.

- Start to copy the Joomla tar.gz on your server using an FTP tool.
- Go in SSH for example to unzip the Joomla tar.gz
- Install joomla with another table prefix - Seet tutorial video 2 www.jms2win.com/tutorial#jms12x-2
- Install JMS - see tutorial video 3 www.jms2win.com/tutorial#jms12x-3
register JMS and install the JMS patches.
- Create your first slave site using exactly the same parameters than in tutorial video CP-04 www.jms2win.com/tutorial#jms12x-CP-04

Once you have created the slave site.
Install the "Article Sharing" in the master website
- Re-install the "Article Sharing" in the slave site or use the master JMS tools to install it in the slave site.

That is all.