For example, how would you let other emails from your other inbox (lets say another gmail) come into that inbox? Like an incoming message goes to that other gmail and you want to see it in your preffered one, how would you do that? Links and Steps appreciated 
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Log in to the Gmail address you want to forward, click settings at the top right of the page. click Forwarding and POP/IMAP, under forwarding, click add a forwarding address button. Add the email address you would like to use instead, and check Forward a copy of incoming mail to, the address you just added.
Note: These instructions will only work if the email address you don’t want to check is a Gmail address, if it’s Yahoo or Hotmail, you can go to settings, Accounts and Import, and click import mail and contacts. This will set up checking of these accounts automatically.
Hope that helps,
Gabe