One of the top use cases for gaming is to come up with game backgrounds whether it's portrait or vertical you can train Layer to learn your art style and help you create new variations out of them.
Select 1024x1024 resolution option.
Make sure all of your input images that you're using to upload to your training is in same size, same aspect ratio and same orientation. Read more about it here.
Make sure you upload as many images as possible to complete to Layer recommended limit 25.
Make sure to review & edit captions to make sure the captions include details that are important to you about these backgrounds.
Make sure to add variations and not repeating images in terms of content.
Backgrounds are one of the best ways to teach Layer your art style as they have a lot of information i.e. lots of different pixels for AI to learn effectively.
Once you created your style make sure to pick the best and most accurate variation on your style's variation page.
Then you can select your style from the drop-down menu and create in your desired orientation and extend the image as desired using "refill" feature or make edits in layers where you see fit.