If you do not have a greenhouse, simply keep plants frost free, with little or no light, and just keep slightly moist, not wet (otherwise they will die). There are various places suitable to keep Fuchsias over the winter. Essentially any cool dark room, loft, basement, garage or similar will do. All you should have to do is check them occassionally to ensure the root does not dry out completely, but if they do need any water give only enough to dampen them. They will not start to shoot new growth until you bring them back into light and warmth in Spring. Try to introduce them back into the light at the end of March or beginning of April if you can, get them outside when you can and bring them into a cool room at night until all risk of frost has past.
When they are brought out of hibernation it's best to repot them in fresh compost.
To repot remove plant from pot, tease out at least half of the old compost; remove any old brown roots leaving as much fine white root as possible. Place in new pot, at least 1 size smaller, add fresh compost, and give adequate water to settle the new compost, but do not drench with water. A good way to avoid overwatering is to place plastic saucers underneath the pots and give water via the saucer. The moisture then goes straight to the root area, this also avoids having a whole pot full of wet soil, which you would have if you watered from the top, this in turn helps against fungi developing such as botrytis. Just remember to empty the saucer after about 5 mins of any water the plant hasn't taken up, they don't like their feet in water. Spray branches and stems with tepid water once per day, this helps new foliage to come back. As soon as new foliage appears start shaping again, ready for the new season.
If you do have a greenhouse, and wish to overwinter in the greenhouse, keep the temperature at a minimum of 40°F or 5°C, with some form of heating just to keep them ticking over until you wish them to come into growth, it's good to give them a bit of a natural rest. When you want them to come back into growth repot as described above and turn up the heat to a minimum 45°F or 8°C. This is best done towards the end of January when the days begin to lengthen as too much heat too soon will cause your plants to be soft and elongated, making them a bit weak. What is needed is a good balance of light and heat to promote short jointed strong growth. Always remember to maintain which ever method of heating you choose, a failure in the middle of a cold night could be disasterous. Likewise ensure your greenhouse is secured to withstand the worst of the weather, any loose glass or vents in a high wind could also spell disaster.
Of course many growers who have a greenhouse but do not wish to afford the expense of heating it all winter or not have sufficient stock to justify heating all winter, choose a combination of both the above methods. Plants are taken indoors and stored before the first frosts then introduced into a heated greenhouse in February or March when the weather becomes a bit milder. This is a most economical way of ensuring you plants are safely protected through the winter and yet brought back into growth early enough to get the best from them the following season.