(I once had the atara replaced on my own tallit. Our regular seamstress was unavailable, so I went to another seamstress I had never used before. I figured sewing on an atara is as easy as falling off a log. The only way to go wrong is if you put it on the wrong side. But I was wrong, somehow she managed to sew it on way off center, even though you have stripes to guide you. And once I had a seamstress not take care to move the tzitzit out of the way; she actually sewed the tzitzit down onto the middle of the tallit. Since those two experiences, I've learned to stick with our regular seamstress. But I still maintain sewing on an atara should be very straightforward. The only thing is it's a bit counterintuitive, because you'd think the atara goes on the same side as the corner reinforcements, but it actually goes on the opposite side. The reason is because when the tallit is worn (assuming it's a size 45 or bigger) you generally flip the sides up onto the corner. They sew the corners onto the underside of the tallit so that they show when the tallit is turned up onto the shoulders.)

But it would really be best if we sent the atara measurements once we know which size tallit you want, because the length may vary.