Configuring vSphere HA is a part of the cluster configuration in vSphere Web Client; click on the EDIT... button in the vSphere Availability area, as shown in the following screenshot:
Configuration in the basic state is straightforward; click on the Turn ON vSphere HA checkbox and then OK:
Under the Recent Tasks area, you can see Configuring vSphere HA on each host, so wait for success on all hosts of the cluster: