You will need:
A T-shirt that fits you well (the snugger the better)
With your chalk mark about an inch in from each shoulder seam.
Connect the marks you just made. Make sure you curve the line just a bit when connecting them.
Cut along the line you just marked on the shirt. Make sure to cut through the front and back of the shirt.
Your shirt should look like this when you finish cutting.
Voila! You're done! Put that sucker on and rock it!