how & when would I take geraniun cuttings for next year

welcome to GoY
when you say geranium do you mean pelargonium, the summer bedding type as opposed to the hardy 'in the garden' all year type.

choose a non flowering stem and cut it off so that there is about 4" of stem. remove all but 4/5 leaves. I pot these stems dry into compost and leave overnight, then i water them. I keep them on the slightly drier side. over the last 20 odd years i reckon i have had about 95% success with this method. you can keep them anywhere where there is no frost. you can do this any time of the year.
i have lifted them in the autumn and wrapped them in damp newspaper and kept in the garage until feb then potted them on. I had 50% success with this.

but there are other ways of doing it and i am sure someone will suggest alternative methods.

I take cutting at any time of the year & just put them in pots with my normal compost (never garden soil!). I never leave them to dry out. I water them immediately after putting several in a pot & they nearly always root fairly quickly.

As for overwintering them, I keep mine in 5" pots on a table in the most sheltered corner of the balcony. I keep them fairly dry & most survive the winter. They even came through the extremely low temps of February this year when we got down as low as 5 or 6 degrees below zero Celsius.

Yes, Balcony's method is also my preferred method (also having a balcony!).