Up to three available missions can be selected. Once selected these missions are "active" and the guild can make progress towards completing those missions.
When the mission is cleared, the guild will earn points. Accumulate guild points to earn rewards. Rewards from cumulated guild points are distributed once a week.
Guild missions of different rarities will appear. Keep an eye out for missions of higher rarities, which award more guild points upon completion.
You can change which missions are active up to three times a week. The weekly counter for how often you can change guild missions will be reset when rewards are received.