• Flash up to date image on sd card.
• Touch ssh in the boot partition

Connect to your pi, then run the following as root:

# Change the system logger
apt-get install -y busybox-syslogd && apt-get purge -y rsyslog
 
# Disable audio, wifi and bluetooth
echo "dtoverlay=pi3-disable-wifi" >> /boot/config.txt
echo "dtoverlay=pi3-disable-bt" >> /boot/config.txt
sed -i 's/dtparam=audio=on/#dtparam=audio=on/' /boot/config.txt
 
# Purge unnecessary packages
apt-get purge -y avahi-daemon bind9-host bluez bluez-firmware dphys-swapfile geoip-database libavahi-common-data libavahi-common3 libavahi-core7 libbind9-161 libdaemon0 libdns1104 libfstrm0 libgeoip1 libisc1100 libisccc161 libisccfg163 liblmdb0 liblwres161 libnss-mdns libprotobuf-c1 pi-bluetooth triggerhappy wpasupplicant
 
# Delete the swap file
rm -f /var/swap
 
# Switch to fastboot
sed -i 's/rootwait/rootwait fastboot noswap/' /boot/cmdline.txt
sed -i 's/console=serial0,115200 //' /boot/cmdline.txt
 
# Disable HDMI on startup
echo "/usr/bin/tvservice -o" >> /etc/rc.local