First you need to add another node into the Windows cluster. This is done with the Cluster Administrator. Run through the wizard to add the machine into the cluster. Your Windows admin can probably do this for you.
After the machine is part of the cluster, run through the SQL installer again and it will give you the option to push the service to the new machine in the cluster. You will need to do this twice, one for each instance. After then you’ll need to patch both instances again to the same patch level you are on currently.