Go to the launcher, click the gear under the image of your pack, open folder, find "resource packs" put it there. If your before a certain version, you can only download texture packs.
I really hate the normal look of minecraft and putting texture packs (recourse packs) on mod packs for me is too confusing and I've seen a lot of textures already made for the mod packs so why not just put them in the game already it would make a lot of peoples experience a lot better.