Don’t worry, I can handle a bit of sarcasm.
And it wasn’t just a ‘blank piece of parchment’. It was a blank piece of parchment stolen from a draw marked ‘highly dangerous’.
To rephrase the main explanation, they probably used trial and error, with the map responding and helping increasingly as they progressed. Remember, when Snape attempted to unlock the parchment, the Map recognized him, and spoke to him, and that was without any password.
So we can safely assume that when Fred and George got the map, they ‘spoke’ with it, and seeing them as kindred spirits, helped them as they tried to find the correct formula. By this I mean it didn’t tell them outright, but gave them hints as they progressed towards the correct formula, but still, fundamentally, using trial and error.