its super easy.
You’ll need each machine to have it’s own copy of ElasticSearch (simply copy the one you have now) — the reason is that each machine / node whatever is going to keep it’s own files that are sharded accross the cluster.
The only thing you really need to do is edit the config file to include the name of the cluster.
If all machines have the same cluster name elasticsearch will do the rest automatically (as long as the machines are all on the same network)
Read here to get you started:
https://www.elastic.co/guide/en/elasticsearch/guide/current/deploy.html
When you create indexes (where the data goes) you define at that time how many replicas you want (they’ll be distributed around the cluster)