Great advice in this thread!
The important thing is that you and the artist work like a team in the way that say, people singing a duet would. It's definitely possible for an writer to work with an artist by just writing out a script that tells them everything and then leaving it at that, but you'll get a better comic if you treat it more like an ongoing collaboration.
Draw on the artist's knowledge of things like character design, page layout, composition, visual storytelling etc. One of the most common issues I find working with writers is that inexperienced writers often try to prescribe what's "a panel" without really knowing much about visual storytelling, so they'll write like:
"PANEL 2: Jake picks up the key and unlocks the door." like... that's at least two panels, my friend. Think about it. Try to draw it, picking up the key... then putting it in the lock and the door opening.
So I often have to go through and tell them "no, no, this is two panels." and "this page has too many panels to give that one panel you want to be full bleed for impact as big as you want it", which is great for the writer learning, but it's a lot of extra work for me.
On your end, as the writer, the thing the artist wants from you is to make sure all of the wider details of the plot are sorted. Stuff like consistent characterisation, a solid storyarc, keeping track of what information is planted and for what payoff, the themes of the story, the pace of the narrative etc. These are things where I, as an artist first, would want to feel like they were in the hands of somebody who I could rely on.
In other words it's like, the Artist is doing the drawing, inks, colours/tone, The writer is writing out the dialogue and managing the plot, but then there's a squishy bit in the middle where you're both handling the way the story is told on the pages together and bouncing off each other. 
