change standalone elasticsearch to cluster

Question

I have a standalone elastic search machine (only one node) that contains a huge amount of data. I need to change it to a cluster. So, I need to add more data nodes to increase scalability. I was wondering how should I do that while preserving the data. I searched everywhere and could not find any docs or tutorials. I know that we can add more nodes to an elastic cluster. My main question is, right now we only have one node for everything (master, data, ingest). How can I configure the cluster to keep the data and add more data nodes as well as a specific master node?