To turn on A/B testing for your campaign, click on the A/B Campaign option during the first step of the campaign wizard (Setup).
Once you select A/B campaign, you will need to provide additional campaign related details like the Test Variable. It can be the Subject Line, Content, From Email or From Name. Once you select your variable, you need to provide a value for each version for this variable. For example, if you choose your Test Variable to be Subject Line, then you need to provide a subject line for Version A and a subject line for Version B. The split setting will ask you to configure how many emails will be sent to your target lists by splitting the total recipients into three groups. Subset A will get the first variation of your campaign, Subset B will get the second variation. Subset C will receive the winning version of both subsets.
If you chose the different content test, you will be prompted to insert both campaign content from the template builder screen.