Step 1: write up a little summary of what happens in a chapter. Like, "Grumpy swordsman travels through forest. Runs into bandits. Has a fight and wins, hooray!".
Step 2: Do a page-by-page written breakdown. Like "Page one: pictures of forest and main character. Page two, main character sees a thing hanging from a tree branch. Page three: he investigates the thing" and so on until I've got a general idea of how many pages the chapter will be.
Step 3: thumbnails/storyboards on paper. Figure out the panel-layouts and roughly where the speechbubbles will go. Also, write the actual dialogue/narrative text - but leave it kind of vague, as I might need to fix and rearrange things.
Step 4: Digital rough sketches, with dialogue. Hammer out more details, fix and fiddle and move around speech bubbles, change my mind about panel-arrangements and how much space I need for things.
Step 5: Draw all the things.
Step 6: Change the dialogue again.
Step 7: Drink tea. Be beset by self-doubt. Pet cat.
Step 8: Change dialogue AGAIN.
Step 9: Leave for a couple of months, because I've got a big old buffer of pages going.
Step 10: Whoops, it's almost time to post! Panic!
Step 11: Change dialogue one last time.
Step 12: Post!
Step 13: Instant regret, urge to fix dialogue once again - but it's too late!
Step 14: Learn to live with it.
Step 15: Rinse, repeat.
But seriously - I do 80% of my writing in the thumbnail/storyboard stage. I don't separate the writing from the drawing/layouts. I need to know what a page will look like before I can decide how much dialogue or how much plot/action I can fit into it. I can't just write "PAGE ONE. First panel: a man enters a room. Dialogue: Blah blah blah. Second panel: picture of the room." and so on and so forth. I need to see it before I can write it.
Some things come together easily, others are really hard. It varies depending on what story I'm telling.