How does the Fed increase the supply of money in an Open Market Operation?

From the information I've gathered, the Fed buys Treasuries from individuals. These individuals will put their money in banks, therefore increasing the quantity of money banks can lend.

However, it would seem the Fed is not really creating money. I mean, when these individuals bought treasury securities, then the supply of money decreased! So the Fed would just be refunding or paying back a "loan"!

Question

How does the Fed increase the supply of money in an Open Market Operation? 

From the information I've gathered, the Fed buys Treasuries from individuals. These individuals will put their money in banks, therefore increasing the quantity of money banks can lend. 

However, it would seem the Fed is not really creating money. I mean, when these individuals bought treasury securities, then the supply of money decreased! So the Fed would just be refunding or paying back a "loan"!