Ok so the goals is to get 10 hearts with a NPC.
You get hearts for every gift someone likes. So I gifted usually candied tree seeds or bouquet of flowers. Till you get 8 maximum, after that you can only raise it with gifts they love. So my NPC loved green smoothies and gave them for the last 2 hearts.
When getting 10 hearts you will get a message from the blacksmith to buy a trincot. This is a heart shaped pendulum, when gifted to your NPC, the achievement unlocks.
Note: not everyone is single and available. In order to get this, the hearts must be red.