You will need a piece of paper from your printer. Lay the front of the pattern on the fold line of your fabric.
This is one of two ways that I use. Draw an outline tapering down from the shoulder. Two necklines are shown. One follows the v-neck but adds a few inches and then gradually joins the line to the shoulder. Another line, above that, alters the v-neck into a round neck. You can get special rulers and tools to make new necklines, but this is how to do it quickly, freehand.
Remember to leave the shoulder seam the same. You will not be raising it at all.
I use my knives instead of pins to anchor down the tissue pattern, as they are easier to remove and move around on a pattern.Insert your piece of paper and trace around the edge of the neckline exactly, with a pencil.
This is what it looks like at this point. The straight edges should be about two and a half inches across.