Sign in to your Gmail account, click the gearwheel icon in the upper right corner of your inbox, and choose Show All Settings.
Then go to the Accounts and Import tab. Locate the View other email accounts line, and click Add an email account.
In the window that opens, enter the email address you wish to import and click Next.
Gmail then offers two options: Associate the account via Gmailify or Import the emails from my other account (POP3). Keep the Associate Account via Gmailify setting so you can use your other email address with Gmail's features. Then click Next.
You should see a new window prompting you to Allow Gmail to access your third-party email information. Validating this permission is essential for Gmail to access and manage your other mailbox.
Return to your inbox. Emails you receive on your other email accounts should now go straight to your Gmail inbox.
Now click on New Message to start writing a new e-mail. A drop-down menu is usually visible at the end of the From line, where your Gmail address is mentioned. You will then be able to select the email address with which you want to send your email from Gmail.
You can repeat this operation for all your email addresses. You will be able to view all messages received in these boxes directly in Gmail, and you will also be able to send messages from Gmail using these addresses.