You should be logged in to the vSphere Web Client as an administrator or user, authorized to alter cluster-level settings and VSAN.
From the vSphere Web Client, navigate to: Home | Hosts and Clusters | Datacenter | Cluster | Manage | Virtual SAN | Disk Management and click on the Claim disks button:
In the resulting box, you can select which SSD and spinning disks to claim on a per-host basis. If you want to claim everything, you can click on the Select all eligible disks button. Once you have selected the disks you wish to use, click on the OK button: