The guides are a bit vague so I’ll put one up:
After getting a Swanana later in the game (can take a few hours waiting for one to turn up) and the Rashberry from the paper Pinata you then have to do the romance requirements for each of them and after this you’ll find the mystery building in the Willys shop.
Use 1 Joy candy each from Ivor Bargin and romance.
Wait for the egg to pop and the achievement is yours.
(Just tap the shovel on the hazelnut, chilli and gooseberry for the Rashberry requirements instant goes rotten, I noticed most comments online said to wait 3 day cycles)
Side note: after gaining a pigxie to tame a mallowolf you have to press the pigxie and wait until the mallowolf destroys it. Otherwise it will continue to fly away.