Make a new folder in your left 4 dead 2 main directory and name it anything you want.
Open the addon in GCFScape and extract the models folder into your new folder.
Point to this new folder in left4dead2/gameinfo.txt


HARDER WAY:
Next recompile whatever model you want the flipped incap for to use a different filepath.
Put this recompiled model into the directory you have the flipped anims model in.
Open hlmv through crowbar or something and load up the flipped anims as its own model.
Load your recompiled model AS A WEAPON

Select your survivor's animation in the sequence list and then load the dual pistols as a weapon as well and you're done happy days

EASIER WAY:
just add
$IncludeModel "flipped_incap.mdl"
to the bottom of your model and recompile it. the new incap anims should be at the bottom of the sequence list

End result, look at how the numbers on his hat aren't ♥♥♥♥♥

if youre survivor model has proportion trick applied then it wont work by default

youll get some abomination instead.

instead open the qc for your model and add
$IncludeModel "flipped_incap.mdl"
at the bottom and then recompile.
now at the bottom of your animation list will be all the flipped incap animations and proportion trick will be applied.
obviously remove this line for your final compile
i think you could probably use this in regular models as well actually whoops