Create logical volume.

lvcreate vg_name -n lv_swap -L 12G

Format the swap space.

mkswap /dev/vg_name/lv_swap

Add entry to /etc/fstab.

/dev/vg_name/lv_swap swap swap defaults 0 0

Enable the LV.

swapon -va

Test.

cat /proc/swaps