I think you're to just dec 2 sts evenly space on the last row as you cast them off. So determine where that should be and when you get there, k2tog instead of k1 and lift the previous st over it.
Though given the dec row has a dec at the beg and end of the row, maybe that's what they meant instead of 'dec evenly'. In which case you work the first 2 sts tog, then one more and cast it off. Just BO as usual to the last 2 sts, then work them tog and cast off.