Welcome to KnittingHelp!
Sounds like the previous instructions gave you the pattern for a series of rows including a decreasse row. Follow that pattern for the number of rows for your size, then dec at the beginning and end of the next row and every 6th row until you have the number of sts for your size. The decrease row is one stitch at each end of the row or 2sts total on the row.
Working 1 st less at beg
and end of dec row as
set work 12 [20:24:30:
Dec 2st as before in next and every
foll 4th [4th:6th:6th:
6th6th] row.45 [47:51:55:
Hope this is the first of many sweaters to come.