***Disclaimer - check your local building code to see if you can actually do this ***
my fireplace has 2 cold air inlets where the air gets warmed up and then flows out from 2 outlets on top so i had to make special openings for it - yours might not need them.
i wont go in the details of specific measuring since these will always be different depending on your ceiling height and fireplace model.
So, first you need material - i ordered a sheet of 16 gauge cold roll steel, 1/8 short of ceiling height and wide enough to cover the fireplace and air inlet/outlet. figure out something that has nice proportions. then i had both sides bent 1" (interior measurements) at 90deg. this adds volume and structure to the sheet.
also ordered 1" x 1" x 3/16 angle bars, 2x floor-to-ceiling lengths plus enough to make the window hole frame and 2x widths of 3/4 square tubing. these will be used across the top and bottom to give it rigidity.
Step one - Make a window frame using angle bar as illustrated in FIG.01
Step two - weld the window frame to the back of the face panel and cut out the opening as illustrated in FIG.02
Optional, if you have inlet and outlets. i created "grills" which are made of 5 horizontal lines on a square pattern of 5" x 5". i used a center punch to mark the beginning and end of each line, then drilled a 3/8 hole at every mark. then using a grinder and a cut-off disk i made horizontal lines connecting the 3/8 holes.
Step three - take a look at FIG.03 for the mounting instructions. drill the mounting holes on the sides of the face panel. here you can get creative with the choice of screws or hole pattern. i made 5 sets of 2 holes, and used brass screws. then brace the angle bars behind the panel's sides as per the installation instructions and mark them at every hole. drill and tap the angle bars.
Step four - i forgot to include these in the illustrations, but i welded a 3/4in square tubing across the top and bottom of the face panel to give it more rigidity. they need to stop short of each sides by about 1/4 inches to give room for the angle bars.
Step five - drill the holes on the other side of the angle bars (the sides that lay against the wall). then you need to measure where these will be installed on the wall. take your time, these need to fit perfectly. once you're all set, screw the angle bars in the wall.
Step six - installation. lay the panel flat on the floor in line with the angle bars, then raise it up and push it over the angle bars. insert the screws in the sides and you're done!
Be careful because the the panel will become hot when you make a fire, and after an hour or so you'll hear a nice "kabong" when it warps under the heat - you'll hear another one when it cools down. its a nice deep and impressive sound, not high pitched like a Chinese gong.
you could paint it using high temp paint (like engine or BBQ paint) or leave it raw like i did. i just wipe it with rust inhibitor every spring.