Creating a Slack App can be initiated by navigating directly to the Create App page.
Fill out all the required fields. Notice there is a workspace selector. This is because every Slack must be associated to a workspace. This is where the App will live, but it does not mean users of the App will have access to that workspace.
If you're following along with Postman, make sure you use Postman's redirect URI: https://www.getpostman.com/oauth2/callback
From the left navigation menu, click OAuth & Permissions to access the OAuth information. To authenticate into the App, you'll need the OAuth Client ID and Client Secret. For this tutorial, we will copy this information into Postman.
Now that we have a Slack App to authorize against, we will setup an OAuth 2.0 client. In this tutorial, we will use Postman's Authorization to walk through the flow.
In Postman's Authorization menu, select OAuth 2.0 for the type
Once OAuth 2.0 is selected as the type, click the Get New Access Token button to open the OAuth configuration modal.
Here we will setup the OAuth client. We'll pull information from multiple sources to complete this form. Find more information on Slack's OAuth flow in the docs.
If you set everything up correctly and pressed Request Token, you should see a familiar Slack authorization window. Select the team you would like to authorize and validate your scopes match what's presented.
After you click Authorize, Slack will return Postman a payload that is saved for future use. You can inspect the payload and find the details of that authorization.