In order to create a story, simply click on the "Shop" tab and click on the top right of your screen on the video icon with a plus. There you will be able to select a product, offer or service and then create a story about it.
You can also create a story from your profile, by clicking on the "add story" button next to your profile icon.
Be mindful that in order to create a story, you need to have create an offer beforehand. If you didn't, clicking to create a story will take you to the offer creation page so you can get started.