Usually the decreases would slant toward the center so a skp at the beg of the row and k2tog at the end. Sometimes they're reversed so they slant toward the edge, but pick one way and do it consistently.
Yes, for the largest size, dec on both ends of the next 15 rows, then on every other row 23 times (which will be over 46 rows) and you will have 42 sts left. Ending with WS means you've just finished it and will BO on the next row, the RS row.
sue- knitting heretic