Click the gear icon in the top right corner and select "All settings". On the left click "Mail" and select Auto forward. Toggle the Auto forward button to the right to turn it on. Add up to four different email addresses separated by a comma and click "Apply changes".
Please note: if you still want to receive a copy your emails in your main account, check "Keep a copy of the message". Otherwise all emails will be forwarded to the email addresses that you entered and you will not receive any emails in this inbox.