This is going to sound counterintuitive, but try writing fanfiction.

Fanfics aren’t necessarily the gorgeous polished final works you’d expect to find on a book store shelf. But what they can do is help put you into a certain mindset.

• How to write in character. You already know these characters very very well: their limits, their fears, their challenges, all of it.
• How to describe a place you can see. You know the setting where these characters move and work and live. Taking the time to describe familiar places gives you enough exercise to try describing unfamiliar places.
• How to move the plot. Is this too complicated for one episode? Then it might not be a short story, it might be a long one. Learning when to keep things simple and when to make them intricate.