Im so scared of seaming! help

Its really pathetic, i love knitting but when it comes to seaming up my project I become a nervous wreck. This is strange considering that i think the matress stitch is amazing and i love watching my seam dissapear perfectly. But I’m always worried about one side not fitting right or bunching up. I’ve just finished a mans cardi which requires no matress stitch in sight. In fact it need reverse stockinette st seaming, which i am really confused on how to do. And on top of that it is a set-in-sleeve, which I’ve never done and am also confused about.
I need someone to hold my hand:sad:

For reverse stockinette, do the seaming on the other side.

Im don’t quite follow. If i do the seaming on the other side, which would be the wrong side, wouldn’t the right side show the seam?

Okay, you’re right, guess that wasn’t well thought out… :??

I think you could still do mattress stitch on the RS in the reverse Stst section.

So do i still put my needle under those ‘lines’ in between the stitches and sew those up? cos on rev stk sts, there aren’t any in between but there are the horisontal ones that look like a bottom loop and a top loop.
Also what does this set-in-sleeve require seaming wise?

Seaming on the right side has you putting the needles between the sts which gets the bottom and top loop on the other side.

For set in sleeves, pin the center top even with the shoulder seam, and the underarm seam lined up with the side body seam. Then ease the sleeve into the space between them.