HOW TO SWITCH:
Some day it will be easier to find that information all in one place, but then, again, that's one more way "third parties" are kept in the background and at a disadvantage.
For now, register via...
A. REGISTRATION FORM. In general terms in those states with Green registration, you can contact the states' Department of Elections. Where I live, you can register at the Department of Motor Vehicles' Driver's License Bureaus (Motor-Voter, remember?), or get registration forms at public libraries and other public offices. You should also be able to register ONLINE (or get the form to do so) in your state.
From here you can get a generic National Mail Voter Registration Form on the Election Assistance Commision (EAC) site. Just follow the instructions on that page. You can also get there by clicking on "Switch NOW!" or Switch TODAY!" on the main page of our site.
B. PARTY SITES. This Switch2Green.org site (admittedly, in the small print) links to a list of states currently offering Green registration. That list contains links to state Green Parties who would also have information.
C. CONTACT GPUS. You can also contact the Green Party National office in Washington, D.C., toll-free, at 866.41-GREEN (866.414.7336).
...don't forget to tell your representative!