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For larger bills like fifties or hundreds, you're going to need a fairly new printer, car paint, Elmer's glue, highlighters, red and blue yarn and phone book paper. I'm not going to get into details, here but I took forensics in college, and we learnt some stuff about fake bills.
Before actually reproducing the bill, spend time examining the real thing; PBS's Nova has a good documentary (which can be found online) which talks almost specifically about the hundred-dollar-bill.it also really helps to look at how clerks react to larger bills (what they do with them to make sure they are legit).
I don't want to break any laws by telling you confidential information, but here are a few helpful tips:
The strip inside the 100 solar bill reads "USA100" upside-down and backwards, and rightside-up and forwards, it glows red under a blacklight.
There are lines on the dollar bill that are meant to mess with conventional printers, causing them to blotch and overlap.
The green symbol on every dollar bill is stamped on separately.
Usually the important things on bills (the numbers, specifically) are raised, and sparkle in the light.
I've said enough, but hopefully you'll be able to make sense out of all this information.