Tune your guitar to open A. In this tuning, the low E, high E and A strings remain the same. The D string is tuned up to E. The G string is tuned up to A, and the B string is tuned up to C#. Use an electric tuner to make sure that you bring your strings to the correct pitches.
Hold strings one and two (the highest two strings) in the fourteenth frets and strum a four beat rhythm on those two strings.
Strum strings four, five and six (the lowest three strings) in four-four time. You play these open, that is you don't hold any of the strings down. You can alternate this by playing the fourth string in the second fret on beats two and four. Continue this for the line, "I went to the crossroad, fell down on my knees." This is your "I" chord, which you will play for 16 beats.
Hold down strings four, five and six in the fifth fret as you strum these strings rhythmically for the repeated line, "I went to the crossroad, fell down on my knees." This is your "IV" chord, which you will play for eight beats.
Play the "I" chord for eight beats.
Hold strings four, five and six down in the seventh frets for the line, "Asked the Lord above 'Have mercy....'" This is your "V" chord, which you will play for four beats.
Return to the "IV" chord for, "now save poor Bob, if you...." Play this chord for four beats.
Play the "I" chord for four beats. This coincides with the last lyric of this line, which is "please."
Repeat this chord pattern (I--IV--I--V--IV--I) for the remaining verses of the song.