1. Fix the unicast discovery list to contain the two nodes you'll end up with. Make sure the cluster name matches.
2. Start the new node.
3. Watch it join the cluster. If it doesn't then check step 1 and repeat 2 until it works.
4. Use the allocation filtering API to ban all shards from the node you will be removing.
5. Wait until there are no shards on that node. You can check using the _cat/shards API.
6. Shut down the node.
You might want to think about getting three nodes and configuring minimum master nodes but I'm sure you've heard that before.