Pendulum Summons are a bit different to how most summons work in Yu-Gi-Oh, and require a bit of preparation, requiring two Pendulum Monsters with a different Pendulum Scale value. That is the number between the red and blue arrows on a Pendulum Monster's information.
Place your two Pendulum Monsters in the Pendulum Zones. Level doesn't matter, they are treated as Continuous Magic Cards. After you have placed them, all you need is one monster in your hand. Select 'Pendulum Summon' and summon any number of monsters from your hand.
The achievement will pop a few moments later.