How to login to multiple accounts with Gmail at once

The how-to guide to sign in to multiple accounts at once. When you have more than one Gmail address, it's very common for personal use and business use, (some people also have their first email address with the weird name), you can use multiple sign-in to switch between accounts without having to sign out and back in again instead of log out then log in then log out, it wastes your time and is quite annoying. Follow the instruction to login multiple accounts in Gmail.