Usual Workflow

The most simple way of generating a bootable image using debootstick is to follow the steps listed below.

1. Generate a chroot environment:

debootstrap --arch=amd64 --variant=minbase focal /tmp/focal_chroot  # use different path in case /tmp/ is mounted noexec

2. Customize it:

$ chroot /tmp/focal_chroot
$ passwd       # update root password
$ [...]        # other customizations
$ exit         # exit from chroot

3. Generate the bootable image:

$ debootstick /tmp/focal_chroot /tmp/img

4. Test it with qemu/kvm.

$ cp /tmp/img /tmp/imgtest       # let's work on a copy
$ truncate -s 16G /tmp/imgtest   # simulate a copy on a 16G-large USB stick
$ kvm -m 2048 -hda /tmp/imgtest  # the test itself

5. Copy the boot image to a USB stick or disk.

$ dd bs=10M if=/tmp/img of=/dev/your-device