how do I file a petition for child visitation in Mississippi if I live in Illinois?

Answer 1

You didn't give us enough information.

Is this a divorce or a paternity case?
Where is the divorce decree filed?
The child lives in Mississippi I presume from your question.

1) Locate an attorney in Mississippi in or near the county where the child lives. If you don't know an attorney there, contact the state bar association in Mississippi and ask them for a referral.

2) Be prepared to pay the attorney a retainer. If the child's custodian (mother/father) is going to contest the visitation, then be prepared for a substantial retainer. ($5,000 would not be excessive)

3) If you are the person who moved from Mississippi (presuming the divorce/paternity suit was there originally) then be prepared to pay most if not all of the travel costs of the child when they are visiting, and/or be prepared to travel to Mississippi.

Good luck!