How to? - Adding email+1@gmail.com for testing purposes

In this post https://premium.wpmudev.org/forums/topic/new-membership-test-plugin-needs-to-be-developed#post-168346, Mason spoke of adding email addresses for testing purposes but I don't understand how to do that.

"For my own, I use the +1 method. You can do this with gmail addresses (and many others) so:
email+1@gmail.com
email+2@gmail.com
email+3@gmail.com

You can really do + anything though, so you could do email+level1membership, etc.

Works pretty well and has the advantage of providing you with the ability to actually receive the email notifications and such."

Is there a tutorial or can someone provide instruction?

Thanks so much for your help.