The first technique to master for this piece involves angling
the slide at about 45 degrees to the fretboard so that the slide only touches
the top D string leaving the rest open.
To start try placing the angled slide at fret 4 on the top string and then pick through all six strings - you will then get a chord of D Major with five open strings plus the slide holding an F# note on the top string (compare this note to the open third string - they should be an octave apart)
Once you've got the hang of holding the slide at the correct angle try playing melody on the top string only- while strumming or picking the open strings - this is the basis of a lot of old style blues where the slide line doubles the vocal melody over a rhythm part on the open strings.
Having mastered this you have to then use your right hand thumb to pick out bass notes while playing the melody on the top string with a right hand finger .
I tend to use finger 2 for the top string and finger 1 for the lower strings but I suggest you use what feels right for you.
The tune is a four bar sequence:
| D / / / | A / / / | G / / / | D / / / |
So lets look at each bar:
The R/H thumb plays a D where the melody is on an F# mostly, giving a D major chord
The R/H thumb plays an A where the melody is on an E mostly, giving an A major chord.
The slide must now cover all six strings at the fifth fret giving a G major chord I use the R/H thumb on the bottom string plus two R/H fingers to get the chord sound and then I use R/H finger 1 to play the phrase on the 4th string.
Observe that the notes with an arrow at fret 3 are actually "blues" notes and should be sharpened. Slide up to about halfway between frets 3 and 4.
Basically open strings 4 then 1 followed by a "lead in" which is a hammer-on on the 2nd string followed by the top string open - I use my L/H index finger to play the hammer-on ( fretted )