I'm going to give you just a brief overview of what an encrypted XML document looks like. As with digital signatures (which use a lot of the same math), the arithmetic is far too complex for most humans to do by hand. You'll always use a software application or library to encrypt documents. Encrypted XML isn't intended to be authored by a text editor like normal XML.
When a document or portion of a document is encrypted, that part is replaced by an EncryptedData element like the one that follows.
<EncryptedData xmlns="http://www.w3.org/2001/04/xmlenc#" Type="URI" MimeType="MIME type" Encoding="URI"> <EncryptionMethod Algorithm="URI"/> <ds:KeyInfo xmlns:ds="http://www.w3.org/2000/09/xmldsig#"> <EncryptedKey> Base64-encoded, ...