List activities that you want your child to do everyday, a few times a week, once a week and perhaps once or twice a month. These do not need to be specific yet, but more general categories such as practice counting or do a craft.
Decide on a number of hours and what chunk of the day will be set aside for a "school day" for your child. For many preschoolers this should not be longer than three to four hours.
Create an opening and closing routine for your school day. The opening activity often includes a welcome song, a calendar activity, a discussion of the weather and a description of the day. The closing activity includes clean up, a reflection of the days activities and answering any lingering questions.
Plan outings, whether for personal matters or school related, in logical times of the day. Perhaps this leads to one or two long days out over the course of a month, but it is less stressful then interrupting numerous days of your child's homeschooling schedule.
Adjust the schedule as needed until you find what works best for your family. You may find that starting and ending your day earlier works best or that working four longer days and one shorter day works best for your family.
Provide options within structure if this allows your preschooler to be more involved in her homeschooling. This would be having three projects laid out and allowing the child to chose which order the projects are accomplished. Make it clear that all three will be done, but allowing the child to choose the order can lead to a better time for everyone.