The sides had to be vented for the shirt to lay flat. The neckline gaped. How? I have re-sewn so many necklines and finally figured out how to make them look good. A few months ago, I sewed the same process with no problems. What happened?
I have no idea, but after a few times wearing the shirts with a gaping neckline, trying to convince myself that it was only three days a week for seven weeks, I couldn't take it any more. So I cut the necklines wider and lower and used bias binding and steamed the neckline and then steamed it some more and things greatly improved. There is still a little gaping, more pronounced on some shirts than others, but now I don't hope nobody sees me wearing these shirts.
I think the lower neckline is more flattering. The reason I didn't do it at first is because after the summer, these shirts get worn to bed to keep me warm and a higher neckline is preferable. I can have my stomach uncovered, but my chest, shoulders, and neck have to be covered or I am cold. Every summer I debate making the alterations to be most flattering for the seven weeks or the most practical for the rest of the life of the shirt.
Anyway, the biggest trick to altering a t-shirt is to start with a size at least two sizes greater than one's normal size. I fit a small, so I buy a large and cut it down.