Within Contentful, a slug is simply a specialized text field. The good news is that it has features like auto-populating based on the title and it validates for uniqueness. To create a slug field, first create a new text field as part of your content model.
Click the "Create and configure" button and then select the "Appearance" tab and choose slug.
You can see below how a slug auto-populates as a user types in the title field.
To learn more about the slug field in Contentful visit this blog post.