You can start with something like a good classic furniture polish, with a generous amount of product it lifts up dirt and grime very well. If you want to be extra careful choose something that is approved for shellac. I know some experts have used basic Pledge furniture polish among other brands. These days Renaissance wax is the most trusted, it gives a very stable finish for. Producst like tis will usually get the job done. For the more difficult clean up jobs I know they use a very well wringed up cloth an, even soapy water to get the grime off. It's generally not recommended for daily cleaning, but the grime needs to come off and you amp up the effort as needed. Be careful with the decals, no hard rubbing. You will run into conflicting advice on this. Sellac is in many ways a very hard, durable and easily to clean finish, but it has some weakneses; it doesn't stand up to prolonged exposure to dampness and alcohol will dissolve it.