How do you achieve quantative tightening? In order to achieve quantative tightening the US Government is letting debt expire and not renewing it.

What does this actually mean in practice?

Presumably once money is made it is essentially impossible to remove it completely from supply?
When a U.S. Treasury bond matures the principle is repaid in money. If the owner of the bond is the Federal Reserve, it can reinvest the money in an asset like a U.S. Treasury bond. If the Federal Reserve wants to reduce the money supply the receipt of the money is coincident with a reduction in the account of the federal government. Since it is computerized they do not have to destroy any currency and coins; they change a number in a computer.