Inside the .zip download is a .minecraft folder and a text file that has instructions on how to install/ how to stop crashes.
To install the mod packs you simply replace your current .minecraft folder with the mod pack one.
Your .minecraft folder is found in Roaming, get there by typing %appdata% into the address bar and pressing enter.
This is roughly what your Roaming folder should look like:
Then Simply back up your .minecraft folder and then delete it from Roaming. Now drag and drop/ copy and paste the downloaded .minecraft into Roaming.
It's really easy to switch to vanilla and back - just use the profiles tab:
If your game crashes on startup, untick JVM arguments:
That's pretty much all there is to it - if you're struggling read the text file or write in the comment section below.