Find a T-Shirt that you really like the fit of. To me for a more feminine shirt, the cap of the sleeve (where the sleeve and shoulder seam meet) should be at the shoulder and not down your arm a ways.
No picture for the next steps. Sorry I forgot to take them. Unstitch the neck ribbing and slightly stretch it. Fold it in half at the seam line and mark it at the two folded edges. Now refold with the marks together and mark the new fold lines. You are actually dividing the ribbing into 4 even sections. Now match each of the four ribbing marks with the four shirt neckline marks. Make sure the seam in the ribbing is at the center back mark of the neck. The ribbing should be a little smaller. Sew the ribbing on with 1/4" seams. The ribbing should be on top. Stretch between pins when sewing. If the ribbing does not stretch enough it will flop down instead of stand up on the shirt. Ribbing should never be larger than 3/4 of the neck opening.