Here are the steps:
1. Log in to your personal gmail account in a browser
2. Go to the Settings gear icon at the top right
3. See all settings
4. Go to the Accounts and Import tab
5. Navigate down to Send Email As section
6. Click on Add another email address
7. The the email address box type your email addres and a + sign and whatever you want to add to it (ex: bobsmith+myamazonorders(at)gmail, but use the actual at sign. LinkedIn keeps trying to link when I attempt to use it in a post.
8. Make sure the "Treat as an alias" box is checked
Now you can use this alias email as a filter or in some cases to sign into a website that is having problems directing you do different sign-in functions using the same email address.
MailMeteor has a good write-up outlining the steps here: https://mailmeteor.com/blog/gmail-alias
Streak has a blog post about the process here: https://www.streak.com/post/gmail-plus-addressing-trick
*kudos to Streak for the cool graphic below this post.