Declare your constructor as usual and add EasySync.SyncManager.RegisterForSyncing(this, GUID);.
GUID could be your PluginInfo.PLUGIN_GUID or any string that is unique enough to not be used by others mistakenly.
For SyncedEntry<T> variables, use the .BindSyncedEntry extension provided by CSync.

public ExampleConfig(Config cfg)
{
  EasySync.SyncManager.RegisterForSyncing(this, GUID);

  clientOnlyVariable = cfg.Bind("Client Only Category", "clientVariable", 1, 
    "An example of a local only config that won't be synced.");

  syncedVariable = cfg.BindSyncedEntry("Synced Settings Category", "syncedVariable", 5, 
    "An example of a synced config that will sync on player joins.");
}

Your class at this point should look something like this:

[DataContract]
public class ExampleConfig : SyncedInstance<ExampleConfig>
{
    public ConfigEntry<float> clientOnlyVariable { get; private set; }
    [DataMember] public SyncedEntry<float> syncedVariable { get; private set; }

    public ExampleConfig(Config cfg)
    {
        EasySync.SyncManager.RegisterForSyncing(this, GUID);

        clientOnlyVariable = cfg.Bind("Client Only Category", "clientVariable", 1, 
            "An example of a local only config that won't be synced.");
        
        syncedVariable = cfg.BindSyncedEntry("Synced Settings Category", "syncedVariable", 5, 
            "An example of a synced config that will sync on player joins.");
    }
}