To get started on the seat, I had to figure the difference of the length of the back rail and front rail (measuring horizontally on just the rails). I then divided that difference in half and marked a line on the front rail of the halved measurent. This allowed me to weave the center of the seat first and then fill the sides in as the last step.
Front rail length: 14"
Back rail length: 12"
Divided by 2: 1"
Marked 1" on each end of the front rail.