In one sentence, describe what the following article is about:

You can use any web browser on PC or Mac. This is the website for Gmail. If you aren't logged in, click your Gmail account, or click Use another account and sign in with the email address and password associated with your Gmail account. It's in the upper-left corner of the Gmail website. It's the white button with a colorful plus (+) sign. There is no need to enter anything in this line. This line will be filled in with the Yet Another Mail Merger add-on. Use the "Subject" line to type a subject for the email you intend to send using the contacts from your spreadsheet. Type the message you intend to send to your contacts in your spreadsheet. Gmail automatically saves your email as a draft every minute or so. You can pull information from your spreadsheet by typing $%[Header]% in your email message. For example, if you want to address the recipient by their first name, you would type something like Hello $%First% in the email message. This will pull the contacts name from the email.

Summary:
Navigate to https://mail.google.com in a web browser. Click Compose. Leave the "Recipients" line blank. Type a subject. Type a message.