“Practice” is a tough word, here. You mostly practice to write comics by writing comics, or by writing other things. And is writing practice, or is writing writing?
I was thrown in at the deep end, I didn’t have time to prepare, I had to learn how to write comics on the fly, which is how I think most pros learn.
It’s not a bad idea to find an actual comics script, there are a ton online, and write a script in that format to learn the methodology.
But writing is writing. It’s all practice for the next thing you write!