Use DiskUtility to create a partition for Arch.

When you’ve booted into arch this new empty partition will be called Apple HDFS+ or something to that extent. Use cgdisk or fdisk to simply delete it so that its now free

To identify what /dev/sdX device is your primary hard drive:

fdisk -l

To proceed with partitioning the device:

fdisk /dev/sdX

We start by creating our root (/) mount point which we want to be about 20 GB

Command: n Partition number: <default 7> First sector: Last sector: +20GB

Next we need to create a swap space for RAM spill over. This should be about 8 GB

Command: n Partition number: <default 8> First sector: Last sector: +8GB