Downloading custom assets

Due to my lack of common knowledge, I keep somehow continuously fucking up every in the way of how I download files. I'll try it how I hear it, but I know I'm doing Something Wrong. Say I want to download the 'Dewm' or 'Frontline' assets. I'll download them, and to be safe, I'll make a file in the custom folder to copy the files to, as well as merging the original files with the models and materials folders in the regular TF directory. I'll load up hammer, and the models will load (On occasion without showing as a bright red, Iconic ERROR text.) So I'll go into the files, delete them and say something like, "Okay, maybe it's just this pack, I'll just continue on without those props and textures and such." But because of me and my dumbassery, I'll have deleted a CRUCIAL file to both programs which causes something to fuck up. So what I really need right now, Is just a detailed guide on how to install models and textures and packs in general. So, as I write this right now, I am reinstalling the game in it's entirety. So, Anyone got a good, Detailed tutorial on downloading assets and such?