My husband would like to adopt but we don't know how to start. Biological father is in and out of prison and I have full custody of the kids.how do we get started?

Easy, you start by hiring a local Family Law attorney. I would not try to do this by myself. The attorney will know how to file the petition and deal with serving the father and getting him to agree or dealing with him not agreeing.

Start by hiring a Family law / Adoption lawyer. THey will know everything that is needed. Lots od speedbumps along the way..you need someone that knows the path well.