"Inc 1 st each side of next row, then every 8 rows 12 times."
It means to increase 1 st on each side on your next row. Then work even without any increasing for 7 rows, then do inc 1 sts on each side again on row 8. Then work even for 7 rows, increase on next row again. You keep increasing every 8 rows until you have increased for a total of 12 times not counting the first row that you increase.
With a total decrease of 26 sts altogether from this instruction.
Use row counter or make a mark with a pen to keep track of each row.
Thank you for visiting and questioning. Hope this helps.