@arbitrarykitten mam..
How do I STOP writing stories in my head, and start writing on PAPER?

The blank white sheet of paper (or word processor screen) can be one of the scariest things in the world. It's very intimidating. Once you set words down on it, they're fixed, at least for the moment. And they're vulnerable. Other people can, at least theoretically, see them. It feels much "safer" to keep them in your head.

The only way to break that spell is...well, write something. It doesn't matter if you think it's terrible. You don't ever have to show it to anyone. You just need to write anything. Start the flow of words onto paper. It doesn't even have to be in the form of a proper story; you can go back and revise and fill that in later. The last paragraph of your question description is a good start. You may want to write down an outline and then come back and fill in later, or just write extemporaneously. Whatever works for you.

Writing is kind of like siphoning liquid. You know, where you put one end of a hose into a container, then suck on the other end until fluid comes out, then put that other end into a container lower than the first one, and the fluid just flows from the one container to the other thanks to suction and gravity with no pump required.

Writing is just like that. You've got to prime the pump and just start writing whatever comes to mind. The more you write, the easier it will be and the better it will be. Just as with siphoning, you may suck a little at first, but before you know it you won't be able to stop. :)
Happy writing....happy posting....happy commenting...happy steeming...
Cheers~~~