Hi Diane, Marketo recommends champion/challenger tests for emails that are used frequently, but I also find that they're one of the easiest ways to set up an A/B test. If you check out
this guide, you can learn more about these tests.
If you don't want to do the champion challenger test, you can also take a look at
this article.
Sorry not to break it down a little more, but they're both multi-step processes, and the Marketo guides are pretty good at describing what you need to do.