Needed to create a bootable Debian USB stick for some maintenance on one of my computers.
Here are the steps so I won't have to search for them the next time :-)

1. Download the Debian live CD image
2. Connect your USB stick and find its device location (/dev/diskX) with:

   sudo diskutil list

3. If needed unmount your USB stick:

   sudo diskutil unmountdisk /dev/diskX

4. Write the downloaded image onto the USB stick:

   sudo dd if=./debian-live-12.5.0-amd64-standard.iso of=/dev/diskX bs=1m