Using Peggo is as easy as entering the name of what you want to record in the main menu's text box. After that, you see a list of all the results; you just have to tap one to go to the recording screen (which is very similar to a YouTube page).
On this screen you just have to hit the Record MP3 button if you want to download the entire song. Alternatively, you can adjust your selection in order to record only a specific bit of the audio. This way, for instance, you can record just the chorus of a song so you can use it as a ringtone for your phone.
You'll find all your music in the downloads tab (which you can customize in the app settings). To play your music, though, you need an external app such as Google Play Music or n7Player, since Peggo doesn't incorporate an integrated media player.
Peggo is an excellent app for downloading music in MP3, as it offers almost the entire library on YouTube and SoundCloud. In addition, the way the app works is unbelievably simple.