A while back, at work, I scavenged a Dell Latitude D530 which was said to be unreliable. Thought I could use it in my garage with a USB inspection camera/probe but haven't yet got around to doing that.
Anyway I've been experimenting with that machine while waiting for an answer.
It seems the Install Alongside other OSs option is just a case of creating some free space and then...
Select free space ADD swap area twice the size of internal ram at the END of the free space.
Select remaining free space, ADD Ext4 journaling file system, mount point /.
Please correct me if I'm wrong.