How do I add USD to Blockchain?

I'm curious, could you explain the process of adding USD to a Blockchain wallet in simple terms? I've heard it's a secure way to manage funds, but I'm not entirely sure how to go about it. Is there a specific platform or service I should use? And what steps do I need to follow to ensure a smooth and secure transaction? Additionally, are there any fees involved, and if so, how much can I expect to pay? Thank you for your help in clarifying this process.