It will differ by distro, but generally for debian, you begin uninstalling systemd by installing something else like SysV init:

apt install sysvinit-core sysvinit-utils
cp /usr/share/sysvinit/inittab /etc/inittab

Then you will need to configure grub by editing /etc/default/grub changing:

GRUB_CMDLINE_LINUX_DEFAULT="init=/bin/systemd console=hvc0 console=ttyS0"

to

GRUB_CMDLINE_LINUX_DEFAULT="init=/lib/sysvinit/init console=hvc0 console=ttyS0"

and then executing update-grub as root.

Then you can reboot so that the system boots off of sysvinit instead and then purge systemd with apt-get remove --purge --auto-remove systemd. This also removes packages that depend on systemd.

Then you pin systemd packages to prevent apt from installing systemd or systemd-like packages in the future.

echo -e 'Package: systemd\nPin: release *\nPin-Priority: -1' > /etc/apt/preferences.d/systemd
echo -e '\n\nPackage: *systemd*\nPin: release *\nPin-Priority: -1' >> /etc/apt/preferences.d/systemd

Depending on if the distro is multiarch, you might also need:

echo -e '\nPackage: systemd:amd64\nPin: release *\nPin-Priority: -1' >> /etc/apt/preferences.d/systemd
echo -e '\nPackage: systemd:i386\nPin: release *\nPin-Priority: -1' >> /etc/apt/preferences.d/systemd

This information was sourced from this wiki dedicated specifically to removing systemd on multiple distributions and replacing it with something else:

https://without-systemd.org/wiki/index_php/Main_Page/