This is an excellent write-up and very quick introduction to creating a cluster in ProxMox with HA.
One thing I'd like to add is that if you're doing this on real hardware nodes with physical networks, consider using a separate physical network for VM/Container traffic vs. Cluster/admin traffic. (at least two physical network paths)
The reason this is important is that mechanism that ProxMox uses to sync cluster metadata is very sensitive to latency. If you're moving a lot of VM traffic around, it is possible for the cluster to think a node is offline, triggering unwanted HA migrations... which can exacerbate a heavy traffic situation. This is especially critical if you're using Ceph for shared storage as it moves a fair bit of data on its own.