I don't know if this is how you do it, but maybe this will help, it works for me.
1) slightly inflate the tube, only enough to barely hold it's shape.
2) Insert the tube into the tire.
3) Insert the valve stem into the rim and one side of the tire.
4) once the first side of the tire is on, start on the second side, starting at the valve stem and work around the rim.
Ian