AssertThat - BDD & Cucumber for Jira is installed as a plugin through Atlassian Marketplace.

Step 1: Installing AssertThat - BDD & Cucumber for Jira is straightforward. Go to Jira Settings → Apps then select find new apps. Alternatively, click on the below link

Marketplace Atlassian AssertThat -BDD Test Automation in Jira

Step 2: Select install.

Step 3: Start configuring Enable & configure AssertThat Plugin