How do I marry an illegal immigrant?

Illegal status notwithstanding, marriage and most family matters are under jurisdiction of the several states, not the federal government, therefore people can marry whomsoever they wish if they comply with the laws of the state in which they intend to contract the marriage. So long as parties to the intended marriage are and can prove that they are of age, single (or if previously married, that they have since divorced their previous partners), then, they can legally marry in the several states. As proof of their respective identities, most states will be satisfied with ordinary forms of identification such as driver license, job ids, passports and the like. Again as stated above, immigration law being federal, stats do not ask for proof of legal status.

If it is just marrying then you may have to speak with someone local if you do not have an attorney. If you are planning on sponsoring this person you would be best served by speaking with an attorney. Many attorneys, such as myself, will even do telephone consultations. Immigration is Federal Law so any attorney that is familiar with immigration can help you.

The same way you would marry anyone ... but once you have married, or perhaps even before you marry, you should both meet with an immigration attorney together to fully discuss the situation. The exact details of each person's situation will determine the options.