Customizing Resource Pack Help!

Hello there, I was wondering if I could get a little info on how to customize a resource pack. I am currently using Jim Stones version of john smith resource pack, but I would like to replace a couple of the textures with ones from the actual John Smith resource pack. I have tried unzipping both packs and replacing the png files with the ones I want, but when I get into game, the only thing that has changed is what it looks like on the action bar. When placed it still uses the original texture. How do I actually go about replacing textures in one pack with textures from another pack? Any help would be appreciated.