Step 3: Prepare the Dome Segments
(Don't worry about drawing on the segments with the marker, because the PETG plastic will be covered on both sides with a thin plastic sheet to protect it. You will want to remove the sheet after you have finished making it.)
Then fold the tabs to 90 degrees ready for joining them.
Don't worry about the top of the dome just yet, because if your cone of silence is anything like mine, the "manufacturing tolerances" will mean the top isn't exactly a perfect octagon, and it's best to leave cutting out the top until you know what shape it will be.
Next you will be joining them together to make a dome like this.