I’m sorry, we don’t have a tutorial on this yet. The trouble with this is that pattern companies don’t provide finished measurements for this area so you kind of have to make a muslin to see what the fit will be and then make your adjustments. And I’m wondering if your issue isn’t necessarily the size of the neckline but maybe you have narrow shoulders. If you have extra fabric in the shoulder seam, I can see how that can cause an issue with your neckline. The process to making this smaller is similar to our decrease/increase the bustline. You make an outline of your bodice pattern. At the shoulder from the armhole direction you go in whatever amount you want to shorten it (so maybe for example, you mark in 1/2″ from the armhole, inside your outline.) Slide your pattern so that the shoulder, armhole meets your new mark. Put a pivot pin at the shoulder/neckline point, pivot pattern to meet underarm seam, and draw new armhole. Then make sure new armhole meets your original under arm seam and then you have your adjustment. I don’t know if this will help your situation but I hope so.