An important prerequisite to creating a cluster is to create one (or more) Domain(s) to use with your cluster(s). See the previous page Manage your domains
Navigate to the Account Settings screen by clicking the Account Settings icon in the upper right corner of the window.
Click "Clusters" in the sidebar, then click the green "+ Create New Cluster" button in the Clusters section. Note that only account owners may create new clusters.
In the form that appears, select options from the dropdown menus for the first four fields. Name your cluster (this step is optional) and verify that the subdomain populated correctly, then click "Create Cluster".