If you are working from a cardigan, I'd say to make the back pretty much as given and then make the front the same way to a point. It depends on what kind of neck you want. With a baby sweater I usually make the front about 2 inches shorter than the back, so you'd make it even more on an adult sweater.
I suggest you take a sweater you own and lay it down and measure the difference between the back at the middle of the neck and the front. You can use that to help you shape the front of your planned sweater. You would make it like the back until it was as long from the bottom up to where you want the neck to start. Then figure out based on your row gauge how many often you will need to decrease on each side to decrease away all the stitches you have to get to the same number of stiches you have for the shoulders on the back. How quickly you get rid of them is up to you and what you want.
What I would do when I get to where I want to begin the neck is to knit across a number of stitches (look at the back piece you have already made and mentally draw where you think the neck needs to be and then work for that result), and then bind off some for the center neck, then work across the rest of the stitches. Work decreases as you want along the neck edge of each side using a separate ball of yarn for each side. Make sure your shoulders end up with the same number of stitches in the front and back.
I hope this helps you a little in your planning.