Take one of your tire tools and open the bead up on the opposite side of the tire next to the stem. This will give you more room to get your hand in there and move the tube into place. Also helps to have the tube partially inflated. By adding some air to the tube helps to prevent pinch flats. Another tip is to use baby powder on the tube. This allows the tube to slide easier against the tire making it easier to get the tube lined up so the stem is straight. Also helps to prevent friction between the tire and the tube.