How do I get partial custody of my child when I agreed the father could have sole custody to end our marriage?

Custody  refers to decision making; joint custody has been the law in Wisconsin since 1987. The law presumes joint custody. I see very few cases where sole custody is granted. If you are referring to placement of your child, that you agreed that the child should be primarily placed with the father, if within 2 years from your initial court orders, you would need to prove imminent physical or emotional harm to modify custody or placement. After two years, there is a presumption that custody and placement should remain as originally agreed upon, but if there is a significant and compelling change of circumstances, you can petition to modify the placement schedule. It would be best to sit down with a custody lawyer in advance to discuss what your chances might be, how long it is going to take and what the expense is going to be.