Go to Matterapp.com or Slack App Directory.
If Matterapp.com, click the CTA button Add to Slack.
Allow Matter to access your Slack workspace.
If installed correctly, Matter will be added to your Slack workspace under the Apps section (see below).
Once added to Slack, you'll need to add Matter to a Slack channel to get started.
See related Help Center Article here.
Note: You may need to have specific Slack workspace permissions in order to add apps to your workspace.