All I can say is that it gets easier. When I was doing my first comic scripts I was taking an evening to do a single page of comics.
Some basic suggestions.
1) Try reverse engineering a comic. Probably a story you like, but any story would do. As in, look at the comic, then write the script for it. It's interesting anyway, but it'll force you to focus on the craft on the page. You should be reserve analysing every comic you read anyway. Panel counts, transition types, devices used, whatever. Learning to write comics ruins reading comics for a while, in as much as learning an instrument warps your listening to music (i.e. I could only hear basslines when I was learning bass, etc).
2) You say you have your stories mapped out. That implies long stories. I'd dump them for now. Write 2-5 page comics. Easier for you to complete, easier for you to experiment with and easier to find someone to draw - and having a script drawn, by any artist, will teach you more than hundreds of pages of comics that weren't drawn. Nothing makes you realise something was a mistake than an artist drawing what you asked for.
3) Okay - let's assume you want to convert your material across of the detailed story. What I'd do - and I normally work from a fairly tight synopsis - is go through the story, writing numbers by each section, each one being an estimate of how much space I'd want to tell that scene. I'd add them all up and compare the number to the number of pages available. And then I'll swear a bit, and then work out where I can compress by using different storytelling methods.
4) Actually, if we're talking that issue based-stuff, go back to those comics and look at them at the higher level. How many scenes are in this issue? You could even write a synopsis for the issue, and then work out how much space they gave each scene. Try to pick apart their pacing to see how it works the way it works. When starting Thor, I picked apart JMS because I wanted it to fit stylistically. He was basically doing 5 4-page scenes an issue. Previously on my Beta Ray Bill stories I was doing 8 2-3-page scenes an issue. These create fundamentally different feeling stories. By looking how each story feels with these different choices you can work out how you want yours to feel.
5) Generally speaking, write for your artist. Scripts are love letters.
6) You know basic accepted these-will-generally-work-rules, yeah? Ending page with something that makes you interested in what comes next. No more than 35 word in a (average sized) panel, no more than 25 in a balloon, no more than 210 on a page. People quote that as 35 on a panel, but they're missing the "average sized" rule. Divide the 210 by how many panels on a page to get a more specific value.
You may note I'm answering a bunch of old questions, as I have a cold, and am not good for anything today. Lucky I'm ahead on scripts, eh?
