Place the ring finger of your left hand (unless you are left-handed, in which case all steps should be performed with your right hand) on the D sting (third from the top when looking down from a playing position), third fret.
Place the middle finger on the G string (fourth from top), second fret.
Place your index finger across both the B and E strings (fifth and sixth from the top) on the first fret. This process of holding a single finger across multiple strings is called a "barre" and is a frequently used technique in guitar playing.
Apply firm even pressure across all strings that you have fretted and strum only the third, fourth, fifth and sixth strings. Do not play the top two strings. You will probably notice that the sound is less than ideal because it takes some practice to learn how much pressure to apply and how to hold both barred strings down without muting one or the other, but continued practice will lead to improved technique.