After the basin frame is constructed and leveled, position the two pieces of 30mil liner. The larger piece is for the basin and the other piece is for the return raceway. It is helpful to dig a shallow channel from the basin frame to the pond edge before placing the raceway liner material. This will help keep the water that flows over the spillway on the liner material as it returns to the pond. If digging is not an option then create a channel with pressured treated lumber, soil, flat stones, or pipe. Do not trim any liner material until the basin is full of water and flowing down the raceway.