Yeah, like everybody else has already said, copy all the files in the item pack into FeralHeart/Media/Items (with Windows, the FeralHeart folder is under Computer/Hard Disk Drives. I don't know how it works with a Mac.)
Also, your item pack should come with something called a ReadMe. In this ReadMe is a code that you need to be able to use the items in-game. Open the ReadMe, copy the code, and paste it into your items.cfg (which is in the main FeralHeart folder). Then you will be able to go into your character creator and equip those items to your characters.
By the way, if you're using Ringo & Delay's item pack, you don't need to do that whole business of opening the ReadMe and copy/pasting the code. Ringo & Delay's items already come with their own items.cfg, so you can just drag it into your main FeralHeart folder and override the old items.cfg.
And another thing...
Items can be a little difficult sometimes. For instance, my character may be wearing caribou antlers and a pearl necklace on my end, but you might see my character as wearing a jack'o'lantern mask and a rainbow necktie on your end. That's because you and I have different items downloaded to our respective FH games, possibly in different orders on the item list in Character Creator. Just keep that in mind.
One way that I try to prevent mix-ups in items is that I download these item packs in this order: Ringo & Delay's Item Pack first, and Tetsu's Wearable Items pack second (with any other item packs after that, you just got to hope for the best). A lot of FeralHearters do it this way, so if you see a character with horns or carrying a keyblade in its mouth, you're probably seeing the right item.
