Head to the General configuration within Confluence by clicking on the cogwheel in the upper-right-hand corner and selecting “General configuration”. Note that Administrator privileges are required to access the General Configuration.

In the menu on the left side, navigate to “Find new add-ons” under the “Atlassian Marketplace” category. Search for “Slack for Confluence”, wait until the entry “Slack for Confluence” is shown and proceed to “Install” the add-on.