Worker Node in Kubernetes
If you are running on VMs or outside of a cloud provider then manually setting this up involves several steps.  The tool you will need is kubeadm (see documentation linked) and you will need to init the nodes and then kubeadm join the nodes to the master.

If you are using a cloud provider, then follow the providers documentation on how to add additional nodes.

A great guide called "Kubernetes The Hard Way" is another good resource for learning how to setup a cluster without any automated tooling.