Help please: sewing buttonband onto front

I have sewn the buttonband to the left front using mattress stitch. I am new at this stitch, but managed. Looks good on outside, but the seam underneath is too bulky, I think. I’m looking for suggestions on how to attach a separately knitted button band. Thanks.

You may have done it all right. If you think the seam is too bulky and want to try to improve it you can sew your mattress stitch a little closer to the edge by a half stitch (or a whole stitch, depending how much you put in the seam). I usually prefer to pick up the button band stitches and knit it on instead of having to sew it. I know there are patterns that say to do it that way but most of the time you can still pick up and knit it on instead. There may be some instances where you can’t change what the pattern says, or you may really think the vertical buttonband is important to the style.