Once you have all the gear you want to level, just tab over to your materials and hold your dismantle button (X or Square) to consume the mote of light as gear experience. The experience gain is affected by telementaries, so use them if you have some. Or buy some, that works as well. This upgrades all of your currently equipped gear, not only your weapons. Remember that motes of light are also used to infuse items. So don't go too crazy.