If you haven't registered your email address with a fax service online, do so before continuing. You will need to use the free trial version of a paid service, so make sure that you cancel your subscription before the trial period ends to avoid being billed. Go to https://www.gmail.com/ on a computer. This will open your Gmail inbox if you're already logged in.  If you aren't logged in, you'll need to enter your email address and password before proceeding. You can't send a fax from the Gmail mobile app. It's a grey button in the upper-left side of your Gmail inbox. Type the number to which you want to send the fax into the "To" section, then type in the extension of the service that you use (e.g., RingCentral).  For example, if you use RingCentral, you would type something like 1234567890@rcfax.com into the "To" section. You must add a country code to the beginning of the fax number if faxing overseas. Type your cover letter text into the "Subject" field. If you don't need a cover letter, you can leave this field blank. Click the paperclip icon at the bottom of the email window, then select your fax's document(s) and click Open to upload them. You can also type information into the body section of the email window if you want to send a text-only fax. It's a blue button in the lower-left side of the email window. Doing so will send your fax to the indicated number.

TL;DR in Chinese:
确保你有在线传真账户。 打开Gmail。 点击写信。 输入传真号码和服务器。 添加信件主题。 上传文档。 点击发送。