Same answer as ever really, just practice slowly. I guess your guitarist told you already but left hand for strings above the note, right hand for strings below. I flatten my picking hand and wrist out completely across the strings at all times.
On 7+ strings depending on the size of your hands, you'll find when on the highest string your hand doesn't reach the low B anymore. That's where the wrist comes into play (at least with the technique I developed, I didn't look at any lessons), else you start to hear some low rumbling building up.