How can I take wrinkley US bills and get them flat like the ones from
a cash machine?

Hello p7willm-ga,

Apparently, you can iron them.  I would suggest trying this out
initially with a one dollar bill, rather than a twenty, in case your
first attempt fails.

The posts in this thread give several tips on how to do this.

"Subject: Ironing isn't just for clothes anymore..." (April 27-28,
2001)
Where's George? [The Great American Dollar Bill Locator]
http://www.wheresgeorge.com/pf_main/messages/988409879_18727.html

And here are a few more ideas:

"Subject: My washing technique" (September 21, 2002)
Where's George
http://www.wheresgeorge.com/pf_main3/messages/1032664268_25243.html

"Subject: YES, kinda, and Huh?" (April 16-18, 2001) [suggestions in
titles of follow-ups]
Where's George? 
http://www.wheresgeorge.com/pf_main/messages/987469257_32468.html

I hope that this information is helpful.


Search terms used on Google:

"wrinkled bill" iron
site:wheresgeorge.com iron
site:wheresgeorge.com ironing

[I tried other searches as well, but the ones I have listed were the
most successful.]

Robert Mitchum ironed his in His Kind of Woman (1951): 

"When I have nothing to do at night and I can't think, I always iron my money."
"What do you press when you're broke?"
"When I'm broke, I press my pants." - Robert Mitchum to Jane Russell in 

(source: http://www.cfoasia.com/archives/9902-36.htm)

Don't forget the spray starch.