Below are a set of quick instructions to build your own ISO.
For a more detailed documentation of the project, see the official documentation.

The best thing you can do, is follow the official documentation for archiso on Arch Linux Wiki.
But the gist is this:

$ mkdir -p ./archiso
$ cd ./archiso
$ cp -r /usr/share/archiso/configs/releng/* ./

We need to update the pacman.conf for the build environment.
The addition will be a custom mirror pointing towards archlife.

[archlife]
Server = https://archlinux.life/$repo/os/$arch
SigLevel = Optional TrustAll

archinstall-gui
mesa
xf86-video-fbdev
xf86-video-vesa
xf86-video-vmware

archinstall-gui is packaged in the archlife mirror.
The other packages are fully optional but dependent on what graphics environment you intend to run the installer on.
This examples simply packages the most common one, VMWare, VirtualBox, Quemu/KVM is tested and appears to be working flawlessly with these packages.

While standing in the archiso folder, create and add the following:

$ mkdir -p ./airootfs/etc/skel
$ cat <<\EOF >> ./airootfs/etc/skel/.zprofile
[[ -z $DISPLAY && $XDG_VTNR -eq 1 ]] && sh -c "startx /usr/bin/archinstall-gui"
EOF

This will auto launch the archinstall-gui.

This is the same setup you'll find on https://archlinux.life.

# cat <<\EOF >> ./airootfs/root/customize_airootfs.sh
systemctl enable archinstall_gui.service
EOF

This is subject to change soon as customize_airootfs.sh is going away.