Say you want to buy something off craigslist, drugs, etc.
It's really easy to print fake bills that seem realistic at first glance. Just use nice thick cream color resume paper or similar. You can either double-side print on one sheet, or you can print both sides of the bill on one face of the paper, then fold the bill over and spray it with adhesive so it stays folded. Get it mildly damp and crinkle it up, working it over with your hands. Hit it with a hair straightener to take the wrinkles back out. That will take the crispyness out of it and roughen up the texture. Continue this process of crinkling and ironing until it feels right.
It will pass for real money at first glance and handling. As a precaution, you want to avoid using your
printer if possible, as they can be traced. Also be weary of fingerprints and any CCTV in the area. The government takes this shit very seriously.
Anyways, have the person meet you at an ATM for payment. This is crucial in the event the person is somewhat savvy and your scam slips. Tell the person you need to get money out of the ATM. Go over to the machine, acting as if you're operating it, careful to keep your head down and out of sight of the ATM camera. Produce the fake bills after your facade of operating the machine, and approach the person.
Count the bills out in your hands so they can see. Fairly slowly, enough for them become impatient, but not so slow they notice anything off about the bills. You want them to be thinking "Yeah yeah, I can count, get the fuck on with it and give me my cash", so when they take it, they are that much quicker to just stuff it into their pocket. If you go too fast, they may feel you made an error or are trying to fuck them over, and will count the bills themselves. This principal may also work in a store when there are several people in line behind you. Fumbling and being inefficient will pressure the clerk to make your transaction as quickly as possible without checking the cash over closely.
You may even want to include a real bill with them, placed on top of the stack, and fold the stack in half as you hand it to them so the real bill blankets the fakes. Keep them distracted with dialog as you hand the money over, and hopefully they stuff the folded wad of money into their pocket without a hitch.
They assume you just extracted the money from the ATM, so even if they are kind of iffy about it, their first thought will be, if even at a subconscious level; 'Well it came out of the ATM, it must be real'. By the time they notice, you're long gone. Hell, your bills might even be so good the person is not only oblivious to the fact they are fake, but even be able to use them to purchase things.
You may be able to do this without the ATM bit, but you stand a greater risk of them calling you out. Good luck.