How do I buy from BitPay?

Excuse me, could you please clarify for me how exactly one goes about purchasing items using BitPay? I'm interested in understanding the process from start to finish, including any necessary steps I need to take to set up an account or LINK a payment method. Is there a specific application or platform I should be using? Additionally, are there any fees or limitations I should be aware of before making a purchase? Your guidance would be greatly appreciated.