Add the `replication` section to the mongod.conf file:

```
$cat /usr/local/etc/mongod.conf
systemLog:
  destination: file
  path: /usr/local/var/log/mongodb/mongo.log
  logAppend: true
storage:
  engine: mmapv1
  dbPath: /usr/local/var/repl-emagine-data
net:
  bindIp: 127.0.0.1
replication:
  replSetName: replocal
```

Restart mongod (I use brew):

```sh
$ brew services restart mongodb
```

Connect with local mongo shell and initiate the replica set:

```
$mongo
MongoDB shell version: 3.2.9
connecting to: test
> rs.initiate({_id: "replocal", members: [{_id: 0, host: "127.0.0.1:27017"}] })
{ "ok" : 1 }
```

Now you'll be secondary, but then it will promote you to primary since you're the only one:

```
replocal:SECONDARY> rs.status
function () {
    return db._adminCommand("replSetGetStatus");
}

replocal:PRIMARY> rs.status()
{
	"set" : "replocal",
	"date" : ISODate("2017-01-06T16:16:27.323Z"),
	"myState" : 1,
	"term" : NumberLong(1),
	"heartbeatIntervalMillis" : NumberLong(2000),
	"members" : [
		{
			"_id" : 0,
			"name" : "127.0.0.1:27017",
			"health" : 1,
			"state" : 1,
			"stateStr" : "PRIMARY",
			"uptime" : 2022,
			"optime" : {
				"ts" : Timestamp(1483719372, 1),
				"t" : NumberLong(1)
			},
			"optimeDate" : ISODate("2017-01-06T16:16:12Z"),
			"infoMessage" : "could not find member to sync from",
			"electionTime" : Timestamp(1483719371, 2),
			"electionDate" : ISODate("2017-01-06T16:16:11Z"),
			"configVersion" : 1,
			"self" : true
		}
	],
	"ok" : 1
}

```

You can tail the oplog at 

```
replocal:PRIMARY> use local
switched to db local
replocal:PRIMARY> db.getCollection('oplog.rs').find()
```

...lots of output here