Open D tuning is an awesome tuning for folk fingerstyle, among other genres.
My song example that i have a tutorial for is Barcelona by George Ezra (click for tutorial!)
In this video, I show you how to get in this tuning and how we can easily play all the common chords in the key of D, mostly with 1 finger!
It can be helpful to have a chromatic tuner, such as this one by TC Electronic, to get into this tuning.
The notes you need to tune to for open D are; D A D F# A D
The D chord can be played by just strumming the open strings (that's the idea of Open D!)
G and A major chords can be played with a flat first finger barring 5th and 7th frets.
The minor chords in this key can be played at 2nd, 4th and 9th frets either just by fretting string 7 at these frets, or with the chord shapes shown later in this video, like we do in my tutorial for Barcelona by George Ezra (click for tutorial!)