The FED has 1 main method of creating currency, and that is buying assets from Banks(almost exclusively). When the FED buys said asset, typically US Treasury bills/notes, it takes the asset, puts the asset on the FEDs balance sheet, and electronically pays the seller buy adding “money” to the bank’s account at the FED.
This is the new money added- and banks can have ~10x the liabilities as assets, so for ever dollar the FED gives to the bank, 10 dollars “appear” in the economy, beacuse the FED takes in a “real” asset- really a part of the gov’t debt, and liqufies it into X dollars.
To reduce the amount of “money” the FED just sells the Treasury Bill/note to a bank, taking in its money.