It's done through the key agreement process itself. If you have the primary private key (SkyKey) you can create the decryption key through the ID input. So if I send something to you, it would be (my SkyKey + your ID "JangoCuni") = unique AES key. To decrypt, you use (your SkyKey + "my ID") to get the same unique shared secret (AES Key). Of course, you use the shared secret value to encrypt the content encryption key.
We tried automating the process of the entire note but we found that people really wanted to put their own language around it. When we hijacked the entire note folks didn't recognize it and it's a lot more "trustable" (is that word LOL) when it comes from a friend with some of their language.