Instructions
Search for good clay. Enter the Moonlight Cave with your hoe in tow and descend to the second level of the cave or lower. Equip the hoe and use it to dig through the bare patches of dirt in the cave floor. You have a random chance of securing a lump of clay that is "good" quality. Pick up the good clay when you find it and it will automatically transfer into your rucksack.
Ship the good clay. At your farm, pull the good clay out of your rucksack and drop it into the shipping bin. Wait one day in in-game time. In the morning, you will receive a letter from a new resident of the village, Saibara. He will have opened a shop in the village called Atelier Saibara.
Befriend Saibara. You will need to increase Saibara's affection points to five hearts before he will offer you the mixing pot. Speak to Saibara once a day and give him gifts to increase his affection points.
Ship nine more good clay. Mine for the clay in the Moonlight Cave and ship them at your farm.
Enter Atelier Saibara. Once Saibara has five hearts and you have shipped a total of 10 good clay (including the first good clay you shipped to get Saibara to move into the village), you will trigger a cut scene when you enter Saibara's shop. He will hand you the mixing pot, which will automatically appear in your kitchen at your farm.