ATC does not have the resources (radar & radios) nor the authority to control you in Class G airspace.

Other than that, IFR in class G is just like IFR in controlled airspace...you must obey all the rules: terrain clearance, altitudes, nav systems, airplane and pilot certification and currency, etc.

Traffic separation is conducted according to the "Big Sky, Little Airplane" theory...you just take your chances. In places with a lot of class G IFR (alaska) there are informal air-to-air frequencies and pilots self-announce when they approach navaids and checkpoints.

If you depart IFR in class G but plan to enter controlled airspace, you will need to file an IFR flight pan and obtain clearance to enter before doing so.