I knit a hat on straight needles and am having trouble sewing it up.
I started using the mattress stitch at the bottom of the hat and have been working my way up. It’s been going good so far, but now I am at the part of the hat where it decreases. I look for a horizontal bar above the last one I used, and it seems like it’s not there because of the decrease. Any advice??
Maybe I’m not the perfectionist that you are, but… aren’t you making it more complicated than it needs to be?
I sew all my knitted things together using the plain old whip-stitch. It stretches, eases into a fairly-non-detectable seam, and holds up for years.
Dot
The bars should still be in the decrease st, just a stitch over. Then the decs themselves will go to the inside of the hat.
I was just thinking of the “mattress stitch” thing…I seamed up sleeves yesterday (I hate that part) I am always afraid they’ll “loosen up” so I did a “crochet seam” which you can see on the outside of the seam…anyway here’s the "dumb" question:( when you do “mattess” do you do the stitching on the “inside” (non public) or stitch up from the outside? 
You do mattress stitch with the right side of your work showing.
You do mattress stitch on the outside, but because of how it pulls the edges together, the seam ends up on the inside.