how do I find a someone else's iphone from my icloud.com account

For privacy and security reasons you would need their Apple ID & password. Yo could then use any internet browser and log into www.icloud.com/find

No way to find someone else's iPhone when you are signed into your iCloud account, but you can sign out of your account and sign in to someone else's iCloud account if you have their Apple ID and Password.

www.icloud.com/find

You can login to their apple account using their Apple ID with their password on the browser.