How to book for a seat of a bus online?

I'm curious about how one might book a seat on a bus online. Could you please explain the process step by step? Do I need to create an account first, or can I just proceed as a guest? Are there any specific requirements or information I need to provide during the booking? Also, how do I know if the seat is available or not? Is there a real-time seat availability check? Additionally, can I choose my preferred seat or is it assigned randomly? Lastly, how do I pay for the ticket? Are there multiple payment options available, and are they secure?