If you need to legally declare custody or if you believe that you should be the custodial parent for your child, you will need to file a motion for child custody with your local court. When you file a motion for child custody, especially if the children aren't already under your care, then the burden of proof is on you. That means that you have to demonstrate why you will create a better environment for the child and why their other parent should not have custody.
Determine what has changed that requires a change in the custody of your child before you go to the county clerk's office to file a motion. You need to prove that the current custody arrangement no longer serves the needs of the child and your family. You can do this by showing that the other parent lacks the resources or the dedication to be a good parent to your child.
Go in person to your local county clerk and ask for the paperwork to file a motion for child custody. This varies by state and locality, so the forms you need may change depending on where you're filing. If your child doesn't live in the same county as you, you should file a motion for custody in the court for their home county.
Fill out the paperwork and make a copy for yourself before it is filed. Filing fees vary depending on your location, as do methods of payment. This information can be found at the website for the court or by contacting the clerk.
Send a copy of the motion through the mail, first-class, to the other parent so that they are aware of the proceedings. Get a Certificate of Mailing from the post office and file it with the county clerk you filed your motion for custody with.
Prepare your evidence and wait for your court date to be assigned.