Dude, listen to the song. You're probably just starting to play, so that's one of the best words of advise I can give you.
If you listen, he's not sliding UP, he's sliding DOWN. It means play 12 and slide down, then play 7 and slide down, so on so forth. How far should you slide you might ask? Whatever sounds good. Sounds to me like he goes almost down to the 2nd fret.
Geniuses are like poets, they speak in metaphors.