You absolutely need to go through the courts and child protectiion services may need to get involved to ensure the guardianship is appropriate, safe for the child and in the best interests of the child, especially if you were once the foster parent. You can probably both jointly file so it is not a combative situation but I would first consult with family law counsel before you move forward.  You need to consider finances, relocation issues, schooling, after-school activiities, medical decisions, and so forth.  The school, for example, is not going to simply agree that a notarized statement makes you the guardian; schools will probably require legal documentation to avoid any legal hotwater.