First things first. The ideas in your head of how a character or scene will look is completely different to your artist. Deal with it. To help you get on the same wavelength however some writing (Shocking I know) is needed.

Dont give the artist everything just the important things to the story; gender, species for example. Gender is the big one. It really helps if you both thing character X (Sam for this tutorial) is male. Sam could be both and for you fantasy writers just writing X'bali and no he/she pronouns you may end up with a character that looks completely wrong. However if you write down every fact, you will have tons and tons of writing and the artist may go awol on you.

Do remember the artist has a real life If they don't get in contact with you and dont write for a while they may be on holiday, or ill. Daily updates on their writing is up to them not you. The artist also knows about deadlines dont remind them, they know.

Do give suggestions Ideas for minor things in the scenes, not necessary but some to fill space. They might not listen to you, but hey its a help. Dont go overboard with this around 3-5 ideas if you can think of them.

Keep to a set format of scriptwriting at the beginning you can mix it about a bit to get a format you are both happy with but dont chop and change otherwise neither of you will know what is going on.

Finally but probably most importantly change the story to fit their style. Dont force your story on the artist if they cant draw certain scenes or really dont like to. You dont like being forced to write in a set way let them draw the comic how they like. The comic belongs to both of you, not one or the other. The writer and the artist must adapt to each other.