Soldering pins underneath a connector
By far the easiest thing to do is simply using a hot plate and a hot air gun. Apply solder paste to the pads (this doesn't need to be very precise, just make sure you don't apply too much). Heat up the board to ~100-150 degrees C on the hot plate, hold the component with thin tweezers (don't use thick pliers, they heatsink too much) and gently use the hot air gun to further heat the part. It will solder very quickly and easily. 

If there are heat sensitive (plastic/plastic-wrapped) parts nearby, put some aluminum foil over top of them so they don't get scalded by the hot air gun.