I know how to use the Watson Assistant API to create a skill, but I also need to create an assistant and add the skill to it. How can I do this with Watson API or SDK?
You can't create a new Assistant by API, you need to use the GUI from Watson Assistant. Concerning the skills, you could create a new one from API but it is really complex. I would suggest to start by creating a skill manually and then using APIs to handle tasks such as backup, switch, adding entities and so on. Essentially, a skill contains all the entities and intents + dialog + some logics. It's a big object in JSON format that is really complex to manipulate programmatically. This way, the user interface do helps the user in those tasks.