https://www.npr.org/sections/money/2010/10/04/130329523/how-... has the full story. The short version was that they made people list prices in both the existing currency, the cruzeiro, and the new one that didn't exist, the real. They then deflated the real at the same rate that the cruzeiro was inflating. (80% per month.) After a while everyone had internalized that the real did not inflate while the cruzeiro did. Then they started making the real and inflation went from 80%/month to approximately 0%/month.
Perhaps we should increase the money supply by 300%, to unlock a 15% gain in productivity!!
Or, they might get pissed off and revolt, tanking the economy by 75%.
One can never really be sure.
For a deeply simplified example: say you printed a bunch of money and handed it to Americans who wanted to buy electronics with the money. Factories in China would happily meet the extra demand for 80" TVs, and for their work they'd receive an influx of dollars, which they'd have to change into yuan to pay their workers and buy supplies.
Currency markets are subject to supply and demand of course, so this influx of people trying to exchange out dollars would make the dollar less valuable relative to the yuan. The devaluing of the dollar on international markets would make all imported goods more expensive, ergo inflation.
See https://en.m.wikipedia.org/wiki/Mutual_credit and p2p cryptocurrency implementation https://docs.offsetcredit.org/en/latest/intro/economic.html
It's not constant, of course. The velocity changes, particularly when the system is under stress.
If the government doubled the money supply instantly, for instance, the real GDP would increase by (say) 5% immediately because people would activate productive capacity that was not there. It wouldn't double, however, because it takes time go to grow.
If velocity were constant prices would (almost) double right away. If the influx of money caused velocity to drop, however, the system could absorb extra money right away.
(Compare that to the theory of the Great Depression that when inequality puts all the dollars in the pockets of misers who save instead of spend, demand collapses.)
Another theory of inflation is that inflation happens if people expect inflation to happen.
If I think that prices are going to go up 5% next year, I am going to ask for a 5% raise, my employer will expect to raise prices 5% next year so it is ok...
People today do not believe in inflation so that governments can borrow and/or print a lot of money without provoking it.
That condition could go on for a long time until the psychology changes, in which case inflation returns.