The decreases won’t fall within the cable. These directions sound like the left front (left as you wear the sweater). Is that correct? If so, on the decreases will always be before the last 24 sts. So knit to the last 26sts, p2tog and then work across the last 24sts including cable and buttonband.
You’ll be knitting fewer sts before the last 26sts in each decrease row because of the decreases.