To securely use Protonmail, you should make your own Protonmail account. Don’t do that on a work computer, and definitely don’t google “how do I leak information?” as that can be traced. Do not use a work device and keep things out of the cloud – directly send documents as PDF or JPG, we'll make sure to strip it of any identifying information.
Please consider carefully whether you can safely leak, and understand the risk you’re taking. We will do everything we can to keep your identity secret, but we can only do so from our end.