Instructions
Attract at least two Whirlms to your garden. Whirlms are the easiest Pinata to get, since you only need to have 10 squares of grass or dirt in the garden for them to appear.
Build a Whirlm house in the garden for 55 coins. This prompts the Whirlms to mate, which is required for a Sparrowmint to become a resident in the garden.
Wait for a Sparrowmint to arrive in the garden. It automatically becomes a resident there.
Plant a buttercup in the garden. You're able to buy buttercup seeds from the store for two coins. Seedos, the gardener who comes to your house daily, will sometimes give you a buttercup seed for free.
Wait until the buttercup develops a flower head. Smack the buttercup with your shovel to remove the flower.
Highlight the Sparrowmint and press "A" to get its attention. Move the cursor over the flower and press "A" to direct the Sparrowmint there.
Wait for the Sparrowmint to eat the flower. When it does, it transforms into a Candary.