It seems like if it says the next thing to do is to decrease you would do it on the next row and then after that work 3 rows and do it on the next row again, which would be the 4th row.
It is easy to decrease in seed stitch if you do it right at the beginning and end of the row. Since you always knit a purl and purl a knit look at the third stitch from the beginning of your row. What will that stitch need to be as you work in your seed stitch pattern? If it is going to be a purl, knit the first 2 tog. If that 3rd stitch will need to be a knit, purl the first 2 tog. That keeps everything in pattern. Work on across the row and when you get to the last 2 work them together as whatever the next stitch needs to be, knit or purl. So if the 3rd stitch from the end was knit the last 2 would be purled tog. If the 3rd stitch from the end was purled then knit the last 2 tog.
Then work 3 rows and on the next row do the decreases the same way, but looking at what you need to do to stay in pattern.