You must wait until your print is completely finished to open the door and remove it. To open the door, press in on the right side of the door, toward the middle, and release. The door should swing open. You will find it easiest to remove your print by taking the build plate out of the printer. Do this by moving the red tabs and sliding the plate outward.
To take the print off the plate, you need to peel off the raft. You cannot use the metal scraper on the Z18, as it will damage the soft build platform top. Try using your hands first, and if you need to, carefully use the plastic scraper to remove the raft.
Clear the build plate of any other plastic bits, return it to the printer, and close the door. The screen will tell you your print is finished; simply press down on the knob to clear the notification.