Don't? Newer bills probably reject stamp ink so bills don't have to returned for destruction and replacement as damaged bills. This costs a lot of tax payer money. I even saw a teller at the bank get chewed out for writing "stack totals" on $2 bills cause she was too lazy to get a rubber band and sheet of scrap paper.
Unlike coins, you can't give a bill a wash to remove the stamp or printed ink from them, you just destroyed that bill when you put where's george on it. It may ahve been interesting when it was a project for a class to determine the spread of money so people know the bill is part of the project, but now you don't need the stamp.
If you want to be a part of where's geroge, then just take every bill you have an enter it all the time. The other people wanting to do it will also be able to take any bill they have and enter all of them all the time. No stamp is needed for those that already want to do it.