Boot from the CD - follow the instructions and choose the .tib image from the external HDD
You will find that the screen menu from the CD is the same screen you get when opening Acronis TI from within Windows.
You ought to test your boot CD to make sure it is okay (has burnt to CD and is working)
Nothing 'automatic' happens when you boot from the CD. You remain in full control at all times to choose options from the menu.