I went up 1 waist size on my jeans size. They recommended that we go up 1 size in the waist and add 1" to the length...I got a 34/32 for me and they fit kind of baggy on me.
I'd order whatever you normally wear in a pair of jeans.... if you like your stuff baggier go up 2 inches on the waist and 1 or 2 inches on the length.