Downloading ISO dev builds
Doing it manually
Head to manjaro Releases review on Github and download your test edition, the ISO files are split in two, match files with the same name ending on (iso.zip & iso.z01)
Then on your terminal you have to join both files before unzipping.
pamac install zip
zip -FF ./manjaro-gnome-22.0-minimal-220913-linux519.iso.zip --out gnome-minimal.zip
unzip ./gnome-minimal.zip
At the end you will get a file ending in (.iso) that you can use in a virtual machine such as Vitualbox, Virt-manager or Gnome Boxes.
Using our tool
pamac install manjaro-iso-downloader
Launch the app and pick a image, you can chose between minimal or full, when its finished you can find your image under (/home/Manjaro-edition-Iso/filename.iso)

Downloading on ARM
If you are on manjaro-arm then the flasher is available to you, not available on x64.
pamac install manjaro-arm-flasher
Chose your device, edition, version and target media drive, it will download the image and flash it making it very convenient to use.
If you prefer to do it manually then head to the ARM release Github repository.
