Here are the steps to create a Dynamic Playable:
Go to your Creative Library and choose the app and concept. You will see a list of your creatives for that concept.
Click on the "+ Dynamic Playable" button in the top-left corner. This will create a new dynamic playable and take you to the edit page for creatives. Under your creative's variable fields, you now have the option to "Add a new variant".
Choose which game variable you want to test. You can select multiple variable fields. Each time you add a variant it increases the number of permutations (unique creative configs) of your ad. The maximum number of permutations you can have is 64.
As soon as you are satisfied with your Dynamic Playable, you can proceed to setting up the creative tests.
To set up your creative tests, please check this article.