You sound sarcastic but it's true. Unless you have government-level resources, you can't make a perfect $20 bill. They don't use paper--water would destroy it--they use a special fabric with a single supplier that only sells to the government. They don't print; they stamp. There are watermarks, security strips, and other features that can't easily be reproduced. Doing it right would be a massive investment, at the risk of being caught and losing everything.