Air Conditioning - Need help with my duct work

I do not know much about HVAC stuff so please bear with me. I am in the process of re-doing my upstairs of my Cape Cod style house. It is a 1 1/2 store house with the finished upstairs that has the slanted ceilings and knee walls on both sides. The upstairs is considerable hotter than downstairs. But I believe this is due to misplacement of my vents and returns. There are currently three vents and three returns. The vents come out from the knee walls and are at the floor level, and the returns are also on the knee walls but up at the top of them which is about 5 feet up the wall right at where the ceiling slants upwards. My questions is where is the best place to put the returns and vents to keep the upstairs the coolest. I can tell the temp difference greatly from the floor and the ceiling. So I am thinking the returns should go on the ceiling to draw out the hot air. Also I am not sure how close the returns and vents can be together so I don't just have cold air sucked in the returns. Any help would be appreciated.

You should have a manuel D done. This will tell you where duct needs to be placed. You should also have a manuel J done this will tell you how much cooling you need.