If you are making your own tallit (or replacing the atarah and/or corners on an existing one), the diagram below will show you how I recommend attaching the pieces:
Fold tallit in half to find the center. Mark this place with a pin. Then fold the atarah in half and insert pin at center. Match up centers and pin the atarah to the tallit. Continue pinning approximately every 1/2.” Inserting the pins at a 90-degree angle (as shown in the diagram below) will result in the atarah lying flatter. Pull the atarah gently to keep the fabric taut.
Stitch as close to the edge as possible with thread that matches the base fabric of the atarah. Remove pins as you sew. Tip: If you are going to stitch on by machine, insert pins upside down so the heads don’t interfere with the presser foot!
On the back of the corners of the tallit, apply a small drop of fabric glue where the hole for the tzitzit has been embroidered. Then punch through the hole from front to back with an awl or small, sharp scissor.
For corner placement, I usually measure in 1/2″ from the side and 1/2″ from the bottom. This is personal preference; if you wish, you can place them right at the edge.
Here’s an example of bad sewing. In the original on the left, the corners were sloppily sewn on. Thru the magic of Photoshop, the one on the right is the way it should be done. Many machines have a special “edge” foot or can move the needle slightly left or right. This will produce a much better outcome.