(NOTE: This assumes that you’re using GRUB.)

sudo vi /etc/default/grub

Change this line:

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"

to:

GRUB_CMDLINE_LINUX_DEFAULT="text"

Save the file, then:

sudo update-grub
 
sudo reboot

When you’re ready to go back to booting into graphical mode, change the value back to “quiet splash”.