"I'm up to the button hole row. It says to K2, K2tog, y2rn, skpo which I've done, then it says work to end, dec as set."
Dec as set means to continue the decrease pattern as you quoted: K15 , k2tog,k3,skp, k22,k2tog,k3,skp,k25,k2tog,k3,skp,k22,k2tog,k3,skp , k15. The numbers between the decreases will change according to where you are in the pattern but the line of the decreases up the sweater will continue. In other words, placing the buttonhole isn't going to change the basic decrease pattern that you've been working. This row will decrease 8sts total.
"Dec on the next row ..." This is a little trickier because it's asking you to follow the decrease pattern but on the wrong side row. So the k2tog will be a purl2tog and the skpo would become a purl2tog through the back loop but that's what you need to do. Follow the same decrease row pattern(underline above) and just keep checking that the decreases line up by flipping over to the front side to look at placement and slant of the decrease. This row will also decrease 8sts.
"...and 1 foll right side rows as set." Now you're back on the right side row and you follow the underlined decrease pattern (again substituting for the numbers between decreases since these keep getting smaller with each decrease row). This row also decreases 8sts total and so by the end of the 3 rows with decrease pattern, you will have decreases 24sts or from 71sts to 47sts.