1) Nominal GDP = Money Spent
2) Nominal GDP = Price Level * Real GDP
3) Money Spent = Money Supply * Velocity
Therefore
4) Price Level * Real GDP = Money Supply * Velocity
So if supply doubles but velocity halves while real GDP remains constant, then there's no effect on prices.
Economists call this "pushing on a string".
The Federal Reserve increases the money supply when it buys securities like Treasury bonds and collateralized mortgages.
This puts more money in the hands of banks, brokers, and wealthy investors who used to own Treasury bonds and collateralized mortgages and now they own money instead.
Banks, brokerages and wealthy investors don't spend money as fast as regular people do. The money sits idle in savings and bank reserves.
That drives the velocity of money down as supply increases, and it's exactly what happened over the last 10 years: https://imgur.com/a/dtVJq
As M2 supply doubled, velocity dropped a lot. That's why the value of a dollar didn't halve even though the supply doubled, tripled, or quadrupled depending how you measure it.