How do I create a batch job in D365FO using X++?

You have a few options, but for new parameter-driven business processes I normally use SysOperation.

The usual structure is:

1. Data contract — parameters.
2. Service class — actual work.
3. Controller — starts the operation and controls execution mode.

Example service:

class MyBatchService
{
    public void process(MyBatchContract _contract)
    {
        // Batch logic.
    }
}

RunBaseBatch is still supported and is used all over standard D365FO, so it is not "wrong." I just would not automatically start new development there if SysOperation fits the requirement better.