• Flash the SD card as usual
• Mount it
• Create /boot/ssh
• Place your user name and enrypted pasword in /boot/userconf
• Unmount it

For instance, assming that you wanted to create the user "derf":

pmount mmcblk0p1
touch /media/mmcblk0p1/boot/ssh
echo derf:$(openssl passwd -6) > /boot/userconf
pumount mmcblk0p1