As you start the game, you will receive two sticker packs to open for free. Contained in these should be a plant consumable. If you do not get one, then you will need to purchase the first sticker pack for 1000 coins.
After you have a consumable, start up a game of either garden ops, or gardens and graveyards. ( The former is better as you could end up as a zombie on G&G. Once you start, you will be shown a number of gardens to defend. Choose one, and as you do, you should notice some plant pots appear around the garden. Simply go up to one and press B and it will open a menu of which plants you have available to build.
Once you pick one, the achievement will pop. If you repeat this process another 4 times, you will also unlock the "Cousins" achievement.
Remember though, that each time you build one you lose it when it dies. Also, when going for the cousins achievement it has to be done in one session, so make sure you have 5 consumables when you start.