Don't know if it's the best, but it sure works for us:
I added a 12x22' room on the back of my garage for an 'auxiliary kitchen'. We actually used it as the main kitchen for about 4 months while gutting the main kitchen down to the studs and rebuilding.
Now, it's my meat cutting room. I started by knocking a hole in the back garage wall big enough to put a set of double 30" doors. Cut the existing floor to tie in drain plumbing.
Then poured a slab for the room. Built a flue for a wood cook stove also, as I couldn't figure a way to get one in the main kitchen layout.
Framed it up. Lumber is off my place, as is all lumber used here. Hole in the back wall is for window AC unit that will run the walk-in cooler yet to be built.
Found a used cooler door mounted in frame. Set that into wood framed opening. Rest the cooler is 4" of foam on walls/ceiling, with white fiberglass panels on the inside for a nice washable finish. Floor is tile, with drain in center.
Mounted some big eye bolts in the ceiling, and it will hold 3 hogs, or a full cow/etc. Finished room is about 6x6.
Space to the right of the cooler is a floor-ceiling shelved pantry for canned goods. Main room is tiled with center floor drain, tiled 4' up the walls, so I can hose the entire thing down. Put a 30 gallon electric water heater under the cabinets, and ran a hose connection off it so I can hose everything down with hot water (nice). Put a used wood cook stove out there.
View to other side of room. Work counter, Corian tops (bought some from salvage yard and built the tops myself) Commercial 2 compartment sink (Craig's list), dishwasher for running canning jars (out of the main kitchen). Door/drawers not finished yet in this shot. We use a 3x5 stainless table in the center for cutting.
When not in use for meat cutting/canning, we keep two chest freezers in there on caster bases. They can be rolled out in the garage when the room is needed to work.
For chicken processing, I built a "Whiz Bang" plucker this past year (after years of hand plucking, one of my least favorite jobs). Does a GREAT job.....30 seconds, you have a completely plucked bird.