I think this video really goes best with this DIY since I explain things and show you. I know that’s not for everyone, so photos of the steps are below.
MATERIALS:
Baseboard/Crown Trim https://www.homedepot.ca/product/alexandria-moulding-1-inch-x-6-inch-x-96-inch-mdf-primed-s4s-moulding/1000437855
Corner Pieces https://www.homedepot.ca/product/alexandria-moulding-3-4-inch-x-3-4-inch-x-8-ft-pvc-exterior-corner/1000421398
Mantle Front https://www.homedepot.ca/product/irving-1x8x8-select-pine/1000115708
Mantel Top/Bottom https://www.homedepot.ca/product/irving-1x6x8-select-pine/1000115678
Caulk https://www.homedepot.ca/product/dap-alex-flex-premium-molding-trim-sealant-white-300ml/1000788074
Wood Filler https://www.homedepot.ca/product/dap-woodpro-x-wood-filler-with-drydex-dry-time-indicator-156g/1001235342
Spackling https://www.homedepot.ca/product/dap-drydex-spackling-946ml/1000449417?rrec=true
No More Nails https://www.homedepot.ca/product/lepage-lepage-no-more-nails-all-purpose-construction-adhesive-266-ml/1000115195
TOOLS:
Jig Saw https://www.homedepot.ca/product/ryobi-18v-one-cordless-orbital-jig-saw-tool-only-/1000427005
Caulk Gun https://www.homedepot.ca/product/workforce-9-inch-dripless-caulk-gun/1000729659
BEFORE:
We pulled the firebox insert forward a little which was no problem! it was just resting there and didn’t appear to be glued or anything. This way we can tuck the shiplap boards in behind it and ahve the fire box in front of the shiplap rather than buried behind it. 
Once the first two pieces of the shiplap we cut we started to attach them with a brad nailer. I used a miter saw to cut all of the pieces since that is the only power saw I have. You can use whatever will cut you a straight line! Also, never assume the width is the same all the way up the fireplace, we learned that this varied about 1/4 of an inch so every piece of shiplap we measured the fireplace before cutting.
We had to used a jigsaw to cut the pieces around the mantle, but a table saw might have been a better option if we had one!
Baseboard corners cut at a 45 degree angle
Building up the existing mantle to 8 inches by adding supporting blocks on the top of existing mantle. This is to ensure that the front of the mantle piece can be 8 inches, but the top and bottom are only 6 inches. Then adding smaller pieces to the ends to fill in the gaps. Scroll down to see a photo of how the end looks for a better understanding.
The wood was attached to the existing mantle by using the brad nailer with nails long enough to go through the wood into the existing mantle.
See how the front piece is one continuous piece of wood with no joints!? I wanted to ensure the joints were hidden on the side, making the front the focus/focal point.
Wood filler was used to fill any gaps. The wood filler I used is stainable!
These are the corner pieces we used for the edge of the fireplace to hide any rough edges of the shiplap and make a nice finished edge.
Next we taped off the mantle and stained it. The colour was called briarsmoke.
