I have a tank style shirt that gapes at the arm holes. Does this mean i need to take the shirt in at the sides to make the arm hole smaller or would putting in a bust dart fix this. Any help would be appreciated.

I dunno but I sure would like to know how to do this too......

Put it on inside out, pin it together where ever it takes away the gap. Try pinning under the arm, see if it fits right. Doesn't work, trying pinning in a dart, etc.