Every 'foll alt row' means every other row. Is that for decs or do they still want you to BO? You have to BO at the beg of a row, so it has to be every row. But usually armhole shaping is binding off for the underarms, then dec a stitch at the beg and end of every RS row for a set number of rows or stitches. So I suspect that's where you are.
sue- knitting heretic