I'm Doug, and I'm very sorry to hear of your situation. My goal is to provide you with excellent service today.
You will need to file a Motion for Modification of Custody and ask the court to award you custody of the children.
You would be better off having a local family law attorney do this for you---but if you have more time than money, you can try and do it yourself. Virtually every county has a law library. Ask the law librarian to show you where the books on Pleading and Practice are located. Generally there are many volumes of Pleading and Practice books from which you can pick the proper one for your situation. While the librarian is typically not a lawyer, and cannot practice law, most are quite willing to assist you in locating the chapter in the Pleading and Practice book which you will need.
Once you have found the appropriate pleading, you will note that in addition to the sample pleading which contains all of the necessary structure for your Pleading, there may also be suggestions to make your pleading more persuasive. Prepare your pleading in accordance with the suggested format in the book, inserting the facts of your case as you know them to be. Then all you will need to do is to draft the pleading on your computer, print it out and file the original with the court while serving the other party, or their attorney, with a copy.
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I wish you and yours the best in 2016,