How to passcode lock an app on iPhone 5?

Ok, so when I first got my iPhone 5, I singly locked an app on it.So, every time I open the app, I have to input the pass code. I tried to do it to a few other apps on my iPhone 5 today and I can't seem to find out how to do it.

I have been searching on the internet how to do it and all I find is that it's not possible. I know it's possible, because I did it to an app already.

Are you sure about that? You can't do that on iPhones. The only passcode lock involves locking the phone entirely.

An app or 2 has their own passcode but that's to protect the data in the app itself rather than 1 you have to input to open & use the app initially. Lemon Wallet is 1 such app.