How long are you away for? if you are away for months then you will have to prep them before you go as if you leave until later ie Dec it might have been frosty whilst you are away. once you are back you can check on them and just keep barely damp until spring.

Unless the weather is bad you can leave your geraniums outside for much longer than otherwise thought. I've left mine till November with no ill effects so, as Seaburngirl says, if you're going to be away a long time then you need to take them in. If it's just a week or two they should be fine.

we did this for the first time, last year, and was pleased that they all survived and got used again this year, although one or two looked a bit woody.Bought some new fabulous ones [pelargoniums] this year, so hope to do the same, placing them out of draughts in the workshop, which is unheated and has plenty of light.Any suggestions about pruning them before they go away, or how much water they need, or any suggestions at all would be welcome I think both to Moriartymandy and myself.

I remove the flowers and don't cut them back until the spring and I reduce the watering so they are almost dry. they are drought tolerant plants. I water mine only if they look very floppy and then only a little amount.
the cold is less of a problem if they are dry but cold and wet is the killer. Over the winter I will remove any dry/crispy/dead leaves to prevent any fungal infections. I also keep them apart so air circulates between them, reducing less chance of infection.