You ask your broker for the physical copy of the stock certificate. The buyer asks their bank for some physical cash. They hand you the cash, you hand them the stock certificates. Same as buying or selling anything else, basically.

Of course, for a transaction of a certain value, you wouldn't want to hold the cash or certificate, so instead you write out a contract and sign it at the moment of exchange. Which is the same as buying or selling something else (e.g. car, house) beyond a certain value.