I have not done a lot of sewing ribbing up, but here is my effort.
This picture shows the piece after I sewed a few stitches.
I was trying to show you how you need to pull it together and sew it so that you take half a stitch from each side and make one whole stitch from them so that it looks like the pattern of K1, P1 ribbing continues right across the piece.
The stitch on the left was a bit tricky and I wasn’t 100% sure where I was supposed to place the stitch, but what I did worked. I was working into the middle of the first stitch on the left.
This is where I took the stitch on the right side. I worked into the centers of the first row of knits.
This is how it looked when I had almost all of it done. I didn’t really want to end off the yarn at the top and waste it. It will be pretty easy if both of your sides have the same number of rows and you are able to get the half stitch that should go together from each side. If the number of rows is off you will have to fudge it and I don’t know how perfectly you can get that to turn out. Sewing up knitting can be pretty forgiving, so it may work. Don’t do all the fudging in one short space. At least you usually don’t want to do it that way. It will not take that much fudging to get it too work probably.
There may be other sites where you could get some better pictures and explanations of how to sew ribbing. You might try a search for them, or even start a new thread and ask if folks know of one. Often someone will know of a source for any information needed.
My sewn together halves created a row of knits that were a little smaller than the regular rows of ribbing. If I was using a good wool yarn I could pull the stitches in that area every which way and that would help to even them out. (It even helped with my cheaper yarn–not pictured after stretching) A little wet blocking might help too.
I hope you figure out something that you like and are able to do. Another idea would be to take it to a yarn shop and see if they can help you learn to sew it up. Or if you can’t do it, pay them to do it for you. (last ditch effort) I hope this helps.