Take the third Matzah, and also a kezayit (the volume of one olive) of the Chazeret - which is to be dipped into Charoset. Combine the two [like a sandwich], and say the following: 

Thus did Hilel do at the time of the Bet HaMikdash: He would combine Passover -- lamb, Matzah and Maror and eat them together, as it said: "They shall eat it with Matzah and bitter herbs." 

Now eat them together -- in the reclining position.

haggadah Section: Koreich