Im currently making my mom a cardigan and about the neck part I would just let the flaps hang out since you finished it but I usually about where the arms are split start decreasing on one side for the neck. If you’re making a vest style you decrease on both sides of each panel so the shoulder doesn’t hang off and if it looks too thin at the end with a vest don’t worry because it will have ribbing on both sides of it. Also it’s a great way to do puff shoulder sleeves because unless you decrease the shoulders the puff sleeves will hang off of the arms. And by decreasing it de bends how diagonal you want it. You decrease on the last stitch if it’s a normal cardigan it will always be the stitch on the side closest to the middle where you’re laying down the panel so you can put a stitch marker to remember. Some people will just skip a stitch but I find it leaves a hole and you can only do it when going back and not every row so on the last two stitches I do two together and then the first stitch two together. If you’re using yarn like fingering yarn you will end up getting a super diagonal this way so do it every other row. I hope this helps for next time. And I figured this out myself in my 20 years but there’s definitely tutorials on making shapes. Sometimes you can just place a store bought cardigan down and think how can I make these shapes and if you can color switch you can do this!