Many ....many....many....parents try to work out verbal or hand-written child support agreements that end up backfiring on the one that was obligated to pay child support.  Even though paying child support by agreement, a court could order back payments for the support of the child.... despite the informal agreement.  Even though a bit more tedious, going through the court system will protect you as dad much more in the long run.  If you and her can agree on what the terms are, it will be a very short process of filing the custody suit and having an attorney prepare an agreed order.  Agreed actions are always cheaper than contested ones.  If you want physical custody and she does not agree, you may have to spend a little bit more money for contested hearings, but if this is what you want for your children, this is the time to be thinking about doing it.  Either way, the process starts off the same in both which is filing a custody suit in the state (usually where the child resides).  If you can afford one, consult or hire a family law attorney.  If possible, find one that has multi-state experience.  This could come in handy considering that you and mother live in different states.