Amazing fireplace...HORRIBLE instructions.
I dont know who wrote these instructions, but I'm a pretty smart guy, and i know PhD's that couldn't have installed this fireplace.
All I can say is - read the WHOLE instruction book. Then, tear every page out and reorganize it into logical assembly - from start to finish.
Frame according to plan, but the measurement for the header height is INCORRECT. Make sure your header actually rests a few mm above the top spacers - do not use the MIN clearance they say (they give 2 minimum clearances in the book - 1 at an exact measurement from the base of the unit in the rough framing section, and one stating 0" clearance above top spacers) The rough framing guidelines will not allow you attach the front fire resistant material - you will have nothing to screw it into.
That's just the beginning of the issues with the instruction book...take your time, read it multiple times.