To create a simple Slash command that enables users to store and retrieve data in and from a MongoDB database using Slack Stitch, start by creating a new Slack App, then set up an Atlas Cluster, Database, and Collection. Next, create a Stitch App with an HTTP Service that integrates with the outside world, including Slack. The service's function will receive incoming requests via its Webhook URL and evaluate the text sent to determine whether to stash, list, or remove URLs. Once the Stitch app is created, add a Slash command to the Slack App console, select Install Your app from the left-hand navigation in Slack's app Management console, and confirm your identity on your slack team. With this setup, users can interact with the Slash command to store, retrieve, and delete URLs stored in the MongoDB database.