To format Slack messages using Zapier, start by setting up an app that will trigger your automated workflow, then fill in the necessary information and use Slack as the action step. Use fields to pull in data from previous steps, such as an RSS entry, into your message, and then format it using Slack's formatting language, which includes italicized text, bold text, strike-through text, and code blocks. You can also include links, emojis, and customize the appearance of your bot by giving it a name and icon, and adding features like auto-expanding links, scheduling, and uploading files.