building a multisig transaction using RPC
if you are on testnet/regtest, it is completely ok, to provide "correct" data (no need for masquerading). There is the famous 2-of-3 multisig from Gavin, which provides a step by step approach using RPC. This should give you the idea behind. 

Generally you are correct, you would search for listunspents, take one of the transaction numbers, and see in the transaction, which unspent output you can use (usually it is a 0 or 1 for the first or second). And for sure this transaction should have some amount, that you can spend. Use "decoderawtransaction" to see the details of the tx. When you have created the tx with "createrawtransaction", then it returns a string of data, that can be used as parameter to the "sign" call.