Chords
How to Chord songs

How to chord songs from Radio.
Some simple songs have only three different chords. Folk tunes, 12 bar rhythm and blues, rock songs and traditional tunes often only use the three primary chords to harmonise the song.
Just turn the roman numeral “I” to the starting chord. The red area shows the other two chords. Together they are called the ‘three primary chords’. 
Here's how I've used the Key-Cycle lately to play some of my favourite songs: You too can learn to do this: Firstly try to figure out what key the song is in... Listen for one root note  (a bass note) reoccurring throughout the song. On your Key-Cycle, point Roman numeral “I” to this note. All the other most common chords will be displayed. I then try out a few of them and can easily hear which ones are used in the tune.
The old fashioned rock song “Hound Dog” has been chorded using this method.

Here are a few other examples:

“We belong together” (by Mariah Carey)

“She will be loved” (by Maroon 5)

“Sitting wishing waiting” (by Jack Johnston)