I've been through this last week. The steps were something like this:
1. You may need to remove local snapshots so Disk Utiltity can resize your partition. use sudo tmutil listlocalsnapshots / and then e.g. sudo tmutil deletelocalsnapshots 2019-05-24-191742 for each one (there shouldn't be too many otherwise you may want to write a script to do it)
2. Launch disk utility and add a new partition fo ZFS (formatted HFS journaled).
3. I think an important step is to use gdisk to change the partition type of the HFS partition as per
https://openzfsonosx.org/wiki/Suppressi ... ing_pop-up I installed gdisk using MacPorts but had to reboot in to recovery mode to disable csr to be able to change the partition type. Without this, Mojave would sometimes mount the partition as HFS even after creating a zpool, corrupting the zpool. This seems to stop happening after rI put lots of data on the pool so perhaps just zeroing the entire partition with dd would be enough to stop it deciding it's an HFS partition.
4. Unmount the HFS partition
5. You should then be able to create a zpool as per
https://openzfsonosx.org/wiki/Zpool#Creating_a_pool