How do I convert GBP to USD manually?

Could you please explain to me how I can manually convert British pounds (GBP) to US dollars (USD)? I'm a bit confused about the process and would appreciate a step-by-step guide. Is there a specific formula or rate that I should be using? Also, where can I find the most accurate exchange rates to ensure I'm getting the best conversion possible? I'd like to understand the entire process so that I can do it myself in the future. Thank you for your help!