Do I need to be able to draw?+
No. You write the premise and make the calls; the studio draws every panel in the look you pick, and draws each character from their model sheet so they look like themselves from panel to panel.
How much of the story is mine?+
As much as you want. The premise, the cast, the places and the voice are yours to change on the board, and you can tell the writer what happens next — one issue's worth or a whole pile of ideas.
What if a panel comes out wrong?+
Ask for another take on that panel. Each issue's panels come to your desk to review, and you can redraw the ones that miss before the issue is finished.