How to pay with a crypto wallet?

Could you please elaborate on the steps involved in paying with a cryptocurrency wallet? I'm curious to know how exactly one goes about initiating a transaction, whether it's for goods or services. Is it as straightforward as selecting the desired cryptocurrency, entering the recipient's address, and confirming the transaction? Are there any specific security measures or considerations that one should keep in mind to ensure a smooth and secure payment process? I'm particularly interested in understanding the role of private keys and how they factor into the transaction process.