Family Law Questions? Ask a Family Lawyer Online.
The best instructions or outline I can find are from local court sites like this one: https://www.loudoun.gov/index.aspx?NID=1907
It outlines the law and the steps to take to obtain guardianship over a minor.
I don't know what county you're in, but it is very possible that the court clerk has forms that you can use to file...and jurisdiction-specific instructions.
Also, if you want to be a co-guardian of the child, that request should be makable to the court as well....you would just set out this fact/request in your petition.