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 if your machine cannot sew over them.
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.