I tried a few nursing shirts with my first baby and they were so overwhelming that I just gave up and resorted to either pulling up or unbuttoning a shirt wearing a nursing tank underneath. The ones I tried were full two layer in front with various openings on the under layer and none of them worked right for me. First I am petite, so they were huge to begin with. Second I am very small breasted so it was a challenge to find the opening and get it to stay where I needed it while nursing. BUT I have discovered a nursing style that I am in love with-and it will be easy to convert many of my current shirt and dress patterns for. The empire nursing opening (sometimes called crop style). This is made basically by having two layers over the bust area. The top layer ends under the bust where an empire style would be-but it is not connected in the middle-just at the side seams. The top layer overlaps the bottom half of the shirt and to nurse all you do is pull up the top layer and pull down the bottom layer. You no longer have to have the whole length of your shirt bunched up over your breast-plus you are exposing very little of the breast as you are nursing. The first brand I saw with this style was BOOB. Then as I have been looking around recently I have found some empire nursing styles in Japanese Weekend (during and after nursing), and Motherwear.
There are some wonderful ladies who have already figured out how to alter patterns to sew your own top with this nursing style.
Rostitchery: Overlap Nursing Top -includes line drawn pattern pictures
Rostitchery: Twist Top with Nursing Access -a bit more challenging than the overlap, but very trendy
Emma’s Sewing: Overlap Style Nursing Clothing -Look on the right side under categories and click on nursing or overlap and you will find the post where she alters a pattern for nursing