So after you get to 27sts, the decreases at the front slope would be: work one row and dec on the second row, as you say and then work 5 rows, decrease on the 6th, work 5 rows, decrease on the 6th etc. Continue with the pattern of work 5, dec on the 6th until you're down to 20sts.
For "Cast off 7 sts at beg of next row and foll alt row.
Work 1 row
Cast off remaining 6.", cast off 7sts, work to the end of the row, turn and work back, turn and cast off 7sts again and work to the end of the row. Turn and work to the end of the row, turn and bind off the last 6sts. Each bind off occurs at the beginning of the row on the same edge, probably the armhole edge.