This is an expanded version of This Script which is better for advanced users

Setup

Unfortunately installing Arch Linux isn't as easy as Raspbian or Kali. We need a linux environment and start by partitioning the Micro SD card

   fdisk /dev/sdX

First Partition +150M type 'c' for W95 FAT32 (LBA)
Second Partition the rest and defaults are fine
Write to disk with 'w'

Next we need to format the drives

   mkfs.vfat /dev/sdx1
   mkfs.ext4 /dev/sdx2

Let's move to our working dir and mount the partitions

   cd /mnt/
   mkdir boot/
   mkdir root/
   mount /dev/sdx1 boot/
   mount /dev/sdx2 root/

Depending upon which version of Raspberry Pi your target system is, you'll need to make a choice:

# Raspberry Pi 4 or 3 Aarch64 (64bit)
wget http://os.archlinuxarm.org/os/ArchLinuxARM-rpi-aarch64-latest.tar.gz

# Raspberry Pi 4 - ARMv7 (32bit)
wget http://os.archlinuxarm.org/os/ArchLinuxARM-rpi-4-latest.tar.gz

# Raspberry Pi 3 and 2 - ARMv7 (32bit)
wget http://os.archlinuxarm.org/os/ArchLinuxARM-rpi-2-latest.tar.gz

# Raspberry Pi 1 and 0 - ARMv6 
wget http://os.archlinuxarm.org/os/ArchLinuxARM-rpi-latest.tar.gz

Now to unpack onto the SD Card

   bsdtar -xpf ArchLinuxARM-rpi-$version-latest.tar.gz -C root
   sync

Just one last thing here, this is also the best time to make edits to config.txt

   mv root/boot/* boot
   nano boot/config.txt
   unmount boot root

First Boot

Default User Info:

alarm - alarm
root - root

If you have ethernet connected, you are good to go out of the box practically. For Wireless connectivity, it's pretty simple. Find and connect to your wireless and make a simple name for it in wifi-menu

   wifi-menu
   netctl enable wlan0-home

Let's initialize pacman and get up to date. SSH is usually started, so once connected you can manage from there.

   sed -i 's/#Color/Color/' /etc/pacman.conf
   pacman-key --init
   pacman-key --populate archlinuxarm
   pacman -Syu sudo

Localization and hostname

   rm /etc/localtime
   ln -s /usr/share/zoneinfo/America/Los_Angeles /etc/localtime
   echo 'newhostname' > /etc/hostname

Modify sudoers and create a new user and cleanup the old one.

   nano /etc/sudoers
   # Uncomment %wheel ALL=(ALL) ALL
   useradd -m -G wheel -s /bin/bash newuser
   passwd newuser
   passwd
   userdel alarm  #May need to reboot and login with newuser first if a process is being used

Stuff We Need

Now that we're up and running, it's time to get the important stuff

Yay

The latest and greatest, now that Yaourt is on the outs

   pacman -S git base-devel go
   git clone https://aur.archlinux.org/yay.git
   cd yay
   makepkg -si

Graphical Environment

Xorg

You need all this, I assure you, then choose XFCE or LXDE

   pacman -S xorg xf86-video-fbturbo xf86-video-fbturbo-git xorg-xinit

XFCE

XFCE is my usual go-to, it's light and configurable and gets the job done.

   pacman -S xfce4 xfce4-goodies

LXDE

LXDE is one I'm intrigued on trying out

   pacman -S lxde

SDDM/LDDE

SDDM is the best login manager, basically

   pacman -S sddm #or lxde
   systemctl enable sddm

Let's create a default config and modify it

   sudo sh -c "sddm --example-config > /etc/sddm.conf"
   sudo nano /etc/sddm.conf

We'll modify autologin for our user and desktop. Available session types are in /usr/share/xsessions/

[autologin]

Session=xfce.desktop

User=user

And finally, let's ensure stuff starts appropriately

   echo 'exec startlxde' > ~/.xinitrc
   echo 'exec startxfce' > ~/.xinitrc

Bugs and Fixes

Pacman has been finicky on AArch64 for some reason, traced it down to

   systemctl stop systemctl-resolved.service

Manjaro

And just in case you want all of the Arch and none of the pain, there's Manjaro Pi4 XFCE

References

Arch Linux ARM RPi4, RPi 3, RPi 2, RPi 1
Wikipedia
Helpful guide from github
ArchlinuxArm Repo (mirror)