Step 1 is adding the memmap command line option to your kernel to reserve chunk of memory.
e.g. memmap=20G!4G which reserves 20G of ram after/starting-at 4G.
You can do step 1 either on your existing install, or if you prefer, by modifying the cmdline of a live distro. Depending on distro you may need to install the pmem module? But fedora has it by default.
Once booted (and modules loaded) you'll see a /dev/pmem0 you can use as a block device.