Create a new class for your config. It must have the [DataContract] attribute and inherit from SyncedInstance<T> where T is the name of your config class.

[DataContract]
public class ExampleConfig : SyncedInstance<ExampleConfig>

Add your synced config variables as [DataMember] SyncedEntry<T> where T is the type of your variable. Note the [DataMember] attribute!
If they won't be synced. You can add them as ConfigEntry<T> and without the attribute.

public ConfigEntry<float> clientOnlyVariable { get; private set; }
[DataMember] public SyncedEntry<float> syncedVariable { get; private set; }