I'm more of a technician than an actual musician,

One guy in the band has this skill or talent, he takes a song, let's say it has 3 chords. like something like "Happy Birthday"
He has the ability to add chord changes to make the song more than what it is, it's like chord improvising, but I know it's more of a theory road map that he follows,.

I know that one way would be using roman numeral chord progessions to change form ke to key, but is there a cheat sheet or a place on the net that I can go to that shows, when going from the root (c) (I) to the fifth (G) (V) you should use this and this to give it more of a flow or tension, or to jazz it up?

Anybody understand my babble?

I should have practiced when I was younger