This is probably the most important step. It's really up to you to figure out how to cover your job responsibilities with the schedule you want. If others in your department are looking for better balance, they might be open to a group schedule that would give each person more time off while making sure all work is covered.
Determine which tasks must be completed in the workplace, which can be done on your own schedule, and which can be eliminated or delegated to another person. If you plan to go part-time, you will either have to eliminate certain duties or produce at a slower pace.
Write down your proposal. Ask co-workers with flexible schedules to share any documents they have, or look for templates on the Internet. Be sure to specify how work emergencies and crunch times would be handled.