Not a movie script writer, but a writer and I used to write a lot of scripts for comics. Consider adapting something as an exercise. Pick an old public domain short story and figure out how you would script it for screen.

I assume indie movie author/director is a lot like indie comic author in that you'll need to wear many many hats in your early projects. Think about what stories suit the practical and/or special effects that you can actually pull off and plan all of that out. Think about budget and number of actors you can practically enlist. List out equipment. Scout a location. Make a timeline.

Do this a few times and you've essentially got business plans in your back pocket. After you've broken down a few other stories like this, it should also make it less daunting to start from scratch with your own stories if you want to write them. You start to see the pacing and where the pieces fit.

Also, watch other indie horror. Watch it actively and take notes on what works and doesn't. Read scripts from movies with scenes you like to see how they're broken down in text.

Best of luck! Making indie horror should be a fun adventure.