Increasing and decreasing is one of the most common questions for those learning and practicing on C2C. C2C, short for corner-to-corner, means just that...You go from one corner to the next, so you will always have a square or a rectangular overall shape. So now what?
SQUARE: Increasing and decreasing in a square is simple as this: When you finish the middle row, startdecreasing on both sides! 
No muss...no fuss...
RECTANGLE: Now this is where things get tricky! But, let's try and break it down for you so we don't overthink it. 
1. When you hit the corner row on your shortest side, start decreasing on that side only on your next row. Continue increasing on the other side.
EXAMPLE: You are working on a blanket that 100W x 80H. You will hit the corner row on the height side first. So when you hit row 81, you will not increase on the height side anymore. You will only decrease from here on out on that side. But, you will continue to increase on the width side until you reach the width's corner row.
2. When you hit the corner row on your longest side, start decreasing on both sides respectively. Continue decreasing until you decrease to one c2c stitch, which will be the upper left corner (if you are working diagonally from the lower right side).