(this is my first Instructable. Please be kind :-D )
Step 1: Draw your cutting lines
Figure out how deep and how wide you want your cutout to be. Draw a U shape on the back of the shirt.
Step 2: Cut it Out!
Step 3: Cut it again
Step 4: Links
Decide how long you want the first chain to be. It helps to try the shirt on to see how it fits your body.
Pin one end of the chain to the top of the shoulder. Stretch the rest of the chain over to where you need it to be.
Pin the other end, and try on again to make sure it fits the way you want.
Cut the chain (or unlink it how ever you like). I added two or three links to the end of each chain, its makes them easier to sew later.
Step 5: Measure it out
Place pins at each point(or make some kind of mark so you know where to sew)
Measure out the rest of the chains. they should gradually get smaller as you reach the bottom of the "U".
Make sure they are not too taut or the shirt will not fall correctly when you wear it.