Last July I purchased 3 small fig plants over the Internet. I re potted them into 3 gallon pots where they seem to have done very well. I live in coastal Rhode Island, and while we have not had a frost as of yet I realize I should finalize my winter plans for these plants.
  While my own basement is too warm (plan is to next year is to somehow set aside a cold area down there) my sister has an unheated empty half high basement at her house that she will let me use. It gets cold but not below freezing..My initial thoughts are to put the plants there, cover each on with one of those large paper leaf bags that you can buy at any hardware store in these parts and wait until next spring.
.  My question  has to do with the time frames.When should I do this? When do I bring them back out? Is there any type of pruning I should do first? What about water? Is the plan to cover the entire plants with a large paper bag adequate? Any advice from those more knowledgeable than myself (probably anyone who reads this) would be greatly appreciated.Thanks.
 

If I was you in your kind of climate, I will simply tie together the branches which are very flexible and cover each one with a few layers of burlap and place it in the unheated garage since your winters are not sever as you mentioned.
My rule of thumb for bring out the plants are when the local maple trees start showing leaf buds swelling but then watching for the frost forecast because at that time the fig trees are prone to damage by frost.

For watering, watch the pots so the soil does not get bone dry.