Keep them very dry. Pick off dead leaves and flowers. Only water them very sparingly during mild, dry spells, and then only in the morning. If the winter is not too severe they should survive, but the secret is...keep them dry!

It's hit and miss if they will survive in a greenhouse. If the temperature falls below zero in your greenhouse, which is pretty certain, you will almost certainly lose them because the moisture in the stems will freeze and damage the plant cell structure. This will kill the plants for sure.

If you can't guarantee a temperature above freezing, then it might be better to cut them right back and keep them in a garage or even a cellar. The lack of light shouldn't be a problem as they will be in dormancy.

If you mean Pelagoriums and not the hardy geraniums then I just shake off the compost, hang them upside down in a dark cupboard and they look terrible in March but put on growth quickly. Frost free and dry is key.

Hi, unless they're a named variety that you particularly want to save, I wouldn't bother trying to overwinter them, it would cost you more in heating bills, ensuring that you kept them frost free, than it would to buy fresh plants in spring, Derek.