I'm new to scripting and comics!
For me I write out dialogue and descriptions as I write my plot and outlines since for me since I started writing stories (novels) I have a really vivid imagination and I can clearly see each scene as I write so it comes more naturally, but the drawing part... hence why I'm working with other comic writers and artists, and I'm learning quite a lot from them.
If you already have plot lines divide those into sub plots/scenes - there isn't always a need for too much words since a picture is worth a thousand words! Dialogue should basically only be there to add 'mood' or personality to your characters or add information that drawings would not necessary be able to explained like "names" of the characters, or their immediate thoughts, questions they want to ask others... and such.
I on the other hand since I write novels mostly take core image ideas explain them in "scripted" format and then take one or two of my "dialogues" for each panel since even in Novels it's not always wise to have a character talk more than you show and tell by actions that they do. So for me scripting is the short version of a novel with only the key components.
I'm not really great at explaining things, but from what I understand is you don't know if should draw before dialoguing or dialogue before drawing; it's really up to how well you have your character's personality down that will help you determine what to do, and I have a friend who once told me she didn't know what kind of personality (speech pattern) her character would have, that was until she drew out her characters. When she did I could even tell what the character might say or "act" before my friend even gave me the character's personality sheet (I wrote a short story for her - it was like 2-5 chapters or so each of about 500 words...)
So if you have the image of your characters clearly down try to picture how they might "talk" that can help with dialoguing.
For example: "I thought I told you!"
The way this is written could easily pass for someone who is confident/rowdy/bossy or "queen like" in nature
"I... I thought... I thought I did tell you..."
The way I wrote it now is for someone who might be shy/unsure/timid/scared or even tired? depends on how you draw the character for "tired" specifically it can change again depending on the character self.
Simple words like "mother" "mama" or "mom" can tell you how the character might hold him/herself up around others
"mother" - someone who might be rich/high in social status like a princess maybe... or even someone who wants to sound sarcastic while "mama" - is more fitting of some one young in age or even gentle in nature or naïve even and "mom" is more generalized characters, typical rebel teens or adult talking casually and comfortably with their mother.
So it's really up to you and how you want to portray your characters, plane out the 'scenes' draw them and then see how the different dialogues would affect your character's expressions and personalities. try it with just 1 page and if it worked then great else you know to try dialogue then drawing