I can tell you how I did mine..
I took a normal tie, then folded the bottom piece up:
Then sewed across the bottom. Remember you will need to keep the tie in tied form when you make this..Once you put the cross on it, it's extremely hard to untie, you will need to just loosen to get it it on and off rather than tying it again..
Then make a cardboard rectangle the size of the cross piece you want. After you do that, make a sleeve for it. It will be this:
You will sew up the 2 long sides and one short side. Then you slip the cardboard into it. I would use thinner cardboard that's easy to bend so you can make it more flexible..Then either glue or hand sew the open side shut.
Then attach like so:
To the back of the tie and sew on up and down wise on the edge of the actual tie, it's a little hard because you need to do it without going through the cardboard..But it's what I did..And here's my cosplay: