How to turn off apps on iPhone 5?

I'm going to Italy for the next month, and I have a small data allowance (100 mb). I want to turn off some apps--log out of them so they don't use my data essentially. These include Tumblr, Snapchat, Twiter, and Instagram.

By just logging out of these, will they not use my data in the background? Is there a way to log out of the Snapchat app? Is there something else I can do to prevent these (but just certain apps) from not using data at all?

If you log out and force stop the apps they should not use up your data. Hope this works.

Try double pressing the home key and then swiping up on the app. This closes the app. I don't know if this will work though, but I hope it does

Just get a new one

From the Home screen:
iPhone: Choose Settings > Cellular.
Turn Cellular Data off. This will disable all cellular data traffic until you enable it.