An 'alt row' means every other row, patterned or plain. Start by decreasing on the next row then work a row plain, dec on next row, plain row and dec on next row. That's at the armhole edge only. At the neck edge you begin the decs on the 3rd row which would be a purl row. Then work 2 rows and dec on the 3rd one again, only it would be at the end of a knit row.
You're doing decs at both the armhole and neck edges, but at different rates - every other row for the armhole, and every 3rd row at the neck.