Ok, now that you are done, here's what I do. I have done this on a number of rtw tshirts that stretch out in the neck or ones I sew and screw up and get a gaping neckline. I open up the back seam of the neck ribbing (or whatever is there) on the inside of the shirt. I then put a safety pin through one end of the clear elastic and feed it through the neck piece (like you would with a drawstring). Then I make sure that I have enough in to hold the neck in place and not have it too gathered, (or have it how I want it) and with both ends of the clear elastic on the outside, crossing over each other, I resew the neck band seam stitching in the ditch across the seam, then a few hand stitches closes the seam if need be and it's done. It works great on a thin neck band or ribbing, that is about the same width or a bit wider than the clear elastic.
If you sew the elastic on the outside, it helps to have a teflon foot to help it glide over the elastic, and I also have laid tissue paper over it to also help the foot go, but it makes it hard to see the elastic so it's a slow process as you have to keep checking. I really like the teflon foot.
And one last edit, I never serge fleece, cause I always end up with a mess. And the edges really don't need the serger finish, cause they don't run or fray. But that's just me.