What is the best way to go about joint custody when both parties are in agreement?

Okay, here is what you do.  You have to file a modified parenting plan with the court.  If everything is agreed, there should be no need for a hearing - you both sign it, and a judge will sign it.  the parentiong plan is a pattern form in Washington - you can download the form and fill it out; however, I strongly suggest you get an attorney to do it for you.  It can be tricky and you want to make sure everything is covered correctly.  If you have equal residential time, then child support may adjust as well, which also means a modified order of child support.      

Title 26 in the Washington State Revised Code outlines all of the laws and guidelines about child custody. In this section of domestic law, parents can find out needed information about parenting plans. The Washington Code requires that parents in Washington make a parenting plan, sometimes called a custody agreement, to govern their custody situation. This parenting plan must be accepted by the family court and is used until the child is an adult.

The purpose of a parenting plan, as outlined in the Washington State Revised Code, is: to provide for the child's physical care, maintain the child's emotional stability, provide for the child's changing needs, to encourage the parents to work together instead of relying on the court, and to protect the best interests of the child. This is the standard that all Washington parenting plans must uphold. Parents need to think about each one of these factors as they make a plan.

Title 26 offers a few different ways that Washington parents can fulfill these objectives. The mother and father must create a residential schedule that shows the physical location of the child every day of the year. This is commonly called a custody and visitation schedule. Parents can also add stipulations and provisions that allow them to work together, provide for the child's changing needs, and protect the best interests of the child. The state code requires that parents have specific provisions about how the parents will resolve future disputes and allocation of decision making. A plan will not be accepted unless this information is included.

As mentioned above, one of the purposes of a Washington parenting plan is to encourage the parents to work together. The law stipulates that if the parents are able to cooperate on a plan and submit an agreement jointly to the court, the court looks it over and will most likely approve it. If the parents cannot agree, they must each create a plan separately to the court and prepare to explain how it benefits the child.

So if you agree and can create a plan that takes in to account all the necessary factors then you will be able to reduce court intervention.  Good luck.