I would recommend you to go through a formal and the most legal process and that is a court system so to avoid any kind of hassle in the future, and it's my sincere advice to you.
To Get Legal Guardianship Of A Child, Do You Have To Go Through The Court System Or Can You Fill Out The Forms And Have It Notarized?
I also suggest the same thing. Going through the court system will ensure there is no problems in the future. I myself am trying to do the same thing with my niece and nephew because my sister is really bad on drugs and can not take care of them. So, best of luck to you.
I went to my local county clerks office. They gave me a self-help line and courts websites that have all the instructions and FREE documents. You will want to file these papers through the court to make sure you are a legal guardian. The other documents you can get online for only for Temporary Guardianship (in most states) that are limited and only last for a period up to 6 months. I would try this route and if you state has free self-help they most likely have a free attorney you can consult with. Good luck!