I find it's easiest for me to start opposite the valve stem. That way it's easier to push the tire bead all the way into the center of the rim where, as rumrunn6 says, the diameter is smaller. I just keep working the bead to the center as I keep the tension on the bead and eventually it gets worked to its point of least resistance. Once the tire is on, make sure to push the valve stem up into the tire because it's easy for the reinforcement to get pinched under the bead.