The first thing you need to do is locate your local probate and family court. You will need to go to the court in the county where the child has lived for the past six months or in the court where the child or parents have any previous court orders. This is important because you need to file with the Court that has jurisdiction.
Once you locate that court go to the court and see what papers you need to complete and file to seek temporary custody/ guardianship of your grandchild. You may need a copy of the child's birth certificate or if a parent has passed away a death certificate. You will need to have reason of why you should be the guardian. If the parents consent and sign off on the matter that is one thing. If not be prepared of why you.
Is a parent unfit due to alcohol or drugs? Try to get proof. Letter from Children services, police reports, school reports if child has issues at late or no show, unkempt.
Keep in mind that when you go to court the court will check your history with social services and your criminal record, you too need to be fit to care for a child. One you file and get a court date you serve the other party and go to court.