Is the band worked the narrow way? Say 8 stitches worked for the length of the sweater front? If so you will need to mesh two methods of doing vertical seams. On the St st side you will do a regular mattress stitch seam and on the garter side do a mattress seam that is done on garter. Here is a chapter
from an on-line knitting book. This is the first page of the chapter on vertical seams. The first thing it shows is regular mattress stitch like you use on St st and the last thing it shows in the chapter (I think) is how to do garter sewing.
Another issue with garter and St st is that you may have different number of stitches to go the same distance. You can still get it to work though, just compensate by fudging when you need to.
But I think if I was making your sweater I would prefer to use picked up bands. You don't always have to follow directions. :-) I'd pick up along the buttonband side first, picking up 1, 2, 3, and then skip a stitch all the way up the side and count how many I get. I usually use a needle 1 or 2 sizes smaller than I used for the St st body. Then work my garter border and bind off. Then on the other side I would pick up the same number. And figure out how to space the buttonholes so they would be as even as possible.