At the time I was using OmniROM so I backed up by existing install using twrp
I used the partitioning steps in this thread
http://forum.xda-developers.com/showthread.php?t=2642299
As TWRP doesnt offer as an extensive amount of partitions to format I chose this recovery and kernel
http://forum.xda-developers.com/showthread.php?t=1901191
As far as formatting was concerned i chose the defaults and formatted every available partition except the external SD card
To get back to where I wanted to be I then did a basic install of OmniROM again and just rebooted back into recovery, giving me TWRP again which I then used to restore my backup
I went for a 6GB partition leaving me with a 7GB internal partition. So far have managed to fill about half the internal data partition with a further 1GB of app data stored on the internal SD
The longest part was the waiting on the backup and restores
I would not say it was a painful process, I actually quite enjoyed it, but then I like to fiddle.
When I came to install Nightowl I had NONE of the issues other people reported due to there being insufficient space for apps