My favorite way to get to open G tuning is by using the strings already available on the guitar. First thing I'll tune is my low E to the octave of the fourth string D note by bringing it down a whole step. I do the same thing on the fifth string by bringing it down a whole step to G to match the octave on the third string. The fourth, third, and second strings remain the same. I'll bring the high E string down a whole step and match it to the fourth string (which is a D). So from low to high, we get DGDGBE.
This is a very common tuning, especially in the Delta blues. Jimmy Page was highly influenced by the music of Robert Johnson and other guitar players of that era. Many of those players played slide guitar in open tuning. The technique of playing slide is more than I can cover here, but a few little pointers to remember:
Mute behind the slide with your left hand. Make sure you don't press too hard or too softly on the strings for the slide to work properly. Make sure to place the slide directly over the fret for the notes to be in tune, the slide replaces the fret. A song like this is a great introduction to slide guitar as it's pretty straightforward and doesn't contain any tricky sections often associated with Delta blues.