After triggering the last memory event of a potential marriage candidate, Rowan will visit your house the next morning.
All the final memories' event are triggered in your farm. The marriage candidate will be around your farm at different timings on different day. Speak to them after being good friends with them and the event should trigger.
Rowan tells you to visit the Harvest Godness. Visit her and she will give you a blue feather.
After obtaining the blue feather, simply speak to the marriage candidate whom you triggered the memory event.
Wedding will take place 7 days later after proposing.You will be taken to the town square automatically on your wedding day.
The following day, your sprite tree will grow and after the cut scene, you will gain an extra heart.
Note that it is possible to marry a candidate of the same gender.