How do I add metadata to the blockchain?

Essentially, one of the output scripts should be

OP_RETURN <the data you want to add>

That's it.

Let me give an example. Say you want to add the string "Melons." to the blockchain.

1. Convert to binary
   "Melons." -> 4d656c6f6e732e
2. Add the push-data-onto-stack command at the start. This is the length of your metadata if it's less than 75 bytes, or OP_PUSHDATAx otherwise.
   074d656c6f6e732e
3. Add OP_RETURN at the start
   6a074d656c6f6e732e
4. Put this in the scriptPubKey of an output of a transaction. I wrote a guide on this here.

How do I find it again?

That's... trickier. Bitcoin is not optimized for searching the metadata of transactions.

Possibilities:

• Look through all of the transactions you get with something like bitcointools.
• Use a website like coinsecrets.org.