Hibernate

First set up swap.

Next add resume=swap_partition to kernel parameters.

For example, with UUID add:

resume=UUID=8a1aac0b-487d-48d5-a683-417031d5098a

Initramfs

If using the base hook, add resume after the udev hook in /etc/mkinitcpio.conf:

HOOKS="base udev resume autodetect modconf block filesystems keyboard fsck"

If using the systemd hook, resume isn't needed.