All the .mesh, images and .material files go into your Items folder. There should've also been a Items.cfg file, you need to drag that into your Feralheart folder and replace the existing Items.cfg file. If there wasn't an Items.cfg then there should be a ReadMe.txt that came with the download.
Open the ReadMe (if there is one) and you might see something like this:
[Antlers]
mesh=antlers.mesh
material=antlerMat
bone=forehead
offset_f=0.1 1.5 0.5
rotation_f=-10 180 0
offset_c=0.1 1.4 0.1
rotation_c=-40 180 0
[Aged Antlers]
mesh=antlers.mesh
material=antlersB
bone=forehead
offset_f=0.1 1.5 0.5
rotation_f=-10 180 0
offset_c=0.1 1.4 0.1
rotation_c=-40 180 0
[Garnet Necklace]
mesh=necklace11.mesh
material=NecklaceMatGarnet
bone=neck1
offset_f=0.05 2.1 0.8
rotation_f=-205 0 180
offset_c=0.05 3.4 0.2
rotation_c=-205 0 180
Depending which item pack you downloaded. Copy all of the script/code and open your Items.cfg file with Notepad. You should be able to Right-click>> Open/Open With and browse for Notepad or
Notepad++. Once opened, just paste the code underneath and remember to go File>> Save when you're finished
Making items is a bit tricky to start off with, you need a 3D modeling program. I use Blender, which is free to use. It takes a while to get the hang on 3D modeling. I say download it and practice while watching some video tutorials to get you started
