Using scissors is one way to get the shirt cut the right way. The only risk here is that you will cut too much and make the sure useless. When you get your scissors, you start at the neckline and go down to the top of the shoulder.
Make sure to hem the two sides so there are no fraying or loose threads that can unravel and ruin your work. Then when you sew on the Velcro make sure the smooth side is sewn next to the skin.
The rough side can be sewn on the top of the T-shirt half opposite from the smooth side attachment. To get your new t-shirt on, make sure to put your injured arm first.
Next comes your head and finally your good arm. When taking the shirt off, simply reverse the process. If your sewing scissors are too big for the job, you can always use a sharp knife or a seam ripper to cut the stitches.
Find the easiest way for you to make these cuts. The key will be to go slow and not be in a hurry. The wrong move may mean starting over with a new T-shirt.
Lastly, you can always visit www.renovamedicalwear.com for you post surgery clothing needs.