Designing a good creative for your ad is not as important as targeting it properly; but it's still very important!
The Creatives step is very straightforward:
Create your Tweet in the Message step first
Then you can decide to add a Card to your Tweet - it will be displayed under your tweet, and you can design it further:
Add a name (to save your template if you like it), a Headline (that will be displayed under the picture), and don't forget to add the Website URL you want the users to go to!
Finally, you can add a picture and/or a video, but be mindful of the ratio and size recommendations for these (800 x 418 pixels for 1.91:1 aspect ratio, 800 x 800 pixels for 1:1 aspect ratio (max 3mb)) (you can read the full details here)
Don't forget to Create your creative, and you're ready for the final step: the Settings!