Curren app: You listen to music and get current for it. You can cash out the current to paypal. Additionally, you can increase the earnings by doing surveys, installing games and watching videos.
The app finances itself by advertisements. How much you earn strongly depends on which country you live in. I'm from Germany and I get roughly $30 per month using this app. It seems we Germans are lucky here, because $30 is more than most people get. I'd be interested to hear from you where you live and how much you can earn with current.
Here's how I use the app:
Nothing special. I just put my mobile phone on the desk while I work and turn it on. To have the music earning active, the phone can't be silent, so I turn the sound to a minimum level and plug in a headset which I put beside the phone - no distracting noises. When the phone is not used for a long time, the music earning is turned off, therefore I check the phone every once in a while (1 hour or so).
In the app you can increase your earnings by playing games, doing surveys or watching videos, but I never use them. Except when I'm idle for too long and the music earning turns off - in this case I watch a single video to turn the music earning on again.
Sadly, the app has some annoying features, like a lockscreen. My workaround is to shutdown the app when I'm not using it - in android you can do it by going to "apps" in the phone settings and press the button "force stop".