Lol! I've sewn a lot of stuff. Are you knitting for yourself/friends/family or knitting general sizes for a large amount of different kinds of people? If you're knitting for friends/self/family, you can always make a cheap dress form by having them wear a t-shirt they don't want anymore, then putting duct tape around them where the shirt is so that it fits snug, then cutting up the back, removing it (and the now-ruined shirt) taping it shut, taping the arm and neck holes so the holes are covered (without crimping them) filling it with any kind of stuffing (stuffing, newspaper, whatever) and closing up the bottom the same way you closed the neck/arms. Likewise, you could do it the other way and close up the bottom first. You can use this to make paper pattern pieces and see what shape(s) you'll need to make. You can also take measurements. There should be some in-depth sewing guides or knitting ones online that will help you know where you should measure on a person. With their measurements, you can make a pattern. I'm figuring you're an advanced knitter and know how to increase/decrease and how that looks on the pattern, so you should be able to figure out stuff with the measurements. Also, you need to keep in mind if the item(s) will fit the person's curves or be more straight and just drape on them. Half of construction is design, really.
Btw, here is Burda's online pictorial of how to measure someone for a fit... http://www.burdastyle.com/techniques/how-to-measure-yourself