Use headings to organize sections of the workflow
Use links, images, videos, and attachments to other documents in the step details
Linking to other documentation helps the person using the checklist and provides detailed directions for people who might be new to the process. You want the title of the step to be short - use the details for adding additional instructions for a step.
More is not necessarily better
It's best to start with a simple workflow and let the people responsible for the process develop the workflow, rather than starting with a really long list of steps.
So, start simple and focus on continual improvement and usage of the checklist for optimal results.