To do the baseline test, you should have an established email with which you will use to check the results.
Go to Analytics, Located on the left side toolbar.
Click Create Email A/B Tests.
Click + Create Email Test.
Enter a title for your test.
Under Schedule a Time, set the time and the date for your test by clicking inside the text box.
Click the Select a List menu and select the list to which you want to send the test emails.
Select at least 2 emails for the test.
Click the Use Baseline? Checkbox.
Click the Select Baseline Email menu and choose your baseline email to check the results against.
Click Create Test.