For the most part, this guide: https://www.uxora.com/other/virtualization/55-install-xpenology-dsm-6-2-x-on-proxmox can be followed, but some tweaks have to be made before starting.

Create the VM and modify the configuration as instructed in the Uxora article.
BEFORE booting the VM, edit the grub.cfg inside the synoboot image:
- To mount the GRUB partition: mount -o offset=$((512*2048)) synoboot.img /mnt/syno
- Then edit parts of /mnt/syno/grub/grub.cfg according to this guide: https://xpenology.com/forum/topic/7973-tutorial-installmigrate-dsm-52-to-61x-juns-loader/
  - The vid=0x46f4 OR 0xf400
  - The pid=0x0001 OR 0xf400
  - Reference: https://xpenology.com/forum/topic/23999-proxmox-60-7-jun-103b-ds3617xs-dsm-622-24922-4-kernel-panic/?tab=comments#comment-129273
Then boot the VM, and you should be able to install using the automated installer.
Follow the post install steps in the Uxora article.
Add a new volume in Proxmox and create a storage in Storage Manager
DONE!

How to activate Active Backup features

Note: Must change admin user password to something without any special characters.

Visit the URLs in order: