Write down all your ideas first. When you’re done with that, figure out the big major things you want to happen in your story. Get down the beginning, climax, and ending.
When you’re finished with that, you can refer back to your ideas list. You can pick one of those ideas to be a subplot in one of your episode in order to get the main plot moving forward.
That’s how i do it anyway. There are many different ways to approach writing an episode story