I use the "Something Else method.
You may wish to download a copy of @swarfendor437 's Unofficial Manual for Z15. here: Unofficial Manual for Zorin 15
Actually, when I came to install Zorin 12 Core way back, I followed this thread by Wolfman, which is still on the old forum archive here: How to use Gparted to dual boot with Windows/ Linux - Old Zorin Forum archive