First of all we need to install QEMU. It is as simple as:

sudo apt install qemu

Next download the latest stable version of Slax from https://www.slax.org/. At the time of writing it is slax-32bit-9.11.0.iso (based on Debian 9). (You may also try to get the testing version, based on Debian 10, from here: https://www.slax.org/blog.php)

Give it a try like this:

sudo qemu-system-i386 \
         -enable-kvm \
         -m 512 \
         -boot d \
         -cdrom slax-32bit-9.11.0.iso

The option -enable-kvm makes it run faster, but it also requires sudo. Slax runs well even with less than 512MB memory. We are telling the virtual machine to boot from the CD drive, and to use the Slax iso as a live CD.

We will also need a virtual disk for installing Slax. Let's create one, and then create a partition table on it:

qemu-img create slax.img 2G
echo "o,n,p,1,,,w" | tr , "\n" | fdisk slax.img
fdisk -l slax.img

Now let's start the virtual machine again, with the virtual disk attached:

sudo qemu-system-i386 \
         -enable-kvm \
         -m 512 \
         -boot d \
         -cdrom slax-32bit-9.11.0.iso \
         -drive file=slax.img,format=raw

On the green desktop of Slax we can right-click and open a terminal (or click at the start icon on the bottom). With the command lsblk we can confirm that we have the disk sda of size 2G and the partition sda1. We need to format this partition:

lsblk
lsblk /dev/sda
mkfs.ext4 -L Slax /dev/sda1

To install Slax on /dev/sda1 we just need to copy the directory /slax/ from /media/sr0/ (the mounted CD), and then make the disk sda bootable by running the script bootinst.sh in the /slax/boot/ directory:

df -h
ls /media/sr0/
ls /media/sr0/slax/
cp -a /media/sr0/slax/ /media/sda1/
cd /media/sda1/slax/
ls
cd boot/
ls
./bootinst.sh

Slax is now installed on slax.img, so we can shut down the virtual machine and start another one without the cdrom and the iso file:

sudo qemu-system-i386 \
         -enable-kvm \
         -m 1024 \
         -drive file=slax.img,format=raw

We can now update the system and install new packages (we couldn't do persistent changes on the live system started from the iso file):

df -h
du -hs .
free -h

apt update
apt upgrade
apt install vim

systemctl enable ssh
systemctl start ssh
systemctl status ssh

The last commands are for starting the openssh-server. However, in order to be able to ssh to the virtual machine, we also need to forward a port to 22 when starting it, like this:

sudo qemu-system-i386 -enable-kvm -m 1024 \
         -drive file=slax.img,format=raw \
         -redir tcp:10022::22

Now we can ssh into the virtual machine like this:

ssh -p 10022 root@localhost

The default password (if you haven't changed it yet) is toor (root backwards). Working from a SSH terminal is more convenient for copy-pasting commands.

We need a partition of about 10G for building LFS. Let's create a new virtual disk, and a partition inside it:

qemu-img create lfs.img 10G
echo "o,n,p,1,,,w" | tr , "\n" | fdisk lfs.img
fdisk -l lfs.img

Now we can start the virtual machine again, connecting to it this second drive as well:

sudo qemu-system-i386 -enable-kvm -m 1024 \
         -redir tcp:10022::22 \
         -drive file=slax.img,format=raw \
         -drive file=lfs.img,format=raw

After logging in (with ssh -p 10022 root@localhost), we can verify that the second drive appears inside it as sdb, and we can format /dev/sdb1:

lsblk
mkfs.ext4 -L LFS /dev/sdb1
df -h
df -h /media/sdb1/

Now we need to follow the instructions on http://www.linuxfromscratch.org/lfs/view/stable/chapter02/hostreqs.html to check for the software required on the host system. It is just a copy-paste. The script version-check.sh tells us that some needed tools are missing. Let's install them and check again:

apt install \
    binutils bison gawk gcc g++ m4 make patch python3 texinfo
bash version-check.sh

Next, we need to use /media/sdb1 as the LFS directory, as described on http://www.linuxfromscratch.org/lfs/view/stable/chapter02/aboutlfs.html

export LFS=/media/sdb1

Then follow the rest of the steps and instructions as presented on the book. In the end, when you have to reboot to try the new system, you can actually boot a system that contains only the LFS disk, like this:

sudo qemu-system-i386 -enable-kvm -m 1024 \
         -drive file=lfs.img,format=raw

However the lfs.img disk will appear on the new system as /dev/sda (not as /dev/sdb), so you have to be careful when you set up the GRUB configuration file (http://www.linuxfromscratch.org/lfs/view/stable/chapter08/grub.html) For example use set root=(hd0,1) and root=/dev/sda1.