As no one has answered you I did a search of members and found this from Majeekahead, dated 25 Jan, 2008 it is still relevant and I hope it helps you.
"Geranium cuttings are best done in late summer or late spring, - in my experience, any good potting compost is fine. you can do them one of two ways, either in water on a window sil, or straight into the soil in pots, i never use rooting powder with mine, if you cut them right you should'nt need it, as they release there own rooting hormone. you choose a healthy shoot preferable one that has no flower buds, if it does have buds remove them. you want about 2 - 3" and remove lower leaves and any that are damaged. cut it off straight just below a knode - the roots will shoot from the knode. you then fill a milk bottle or something simular with water and i usually put 3 or 4 in together, but not so that it is too crowded, and the water level wants to be just above the knodes where you want the roots to shoot from. do not change the water just top it up until you have a good root system and then pot into compost as you would do with any young plant. i find this is the best way but you can just put them straight into soil in pots, cut the same way and water them in well but then let them get quite dry before you water again, - keep in a sunny place. i would say this is the wrong time of year to start them off this way (if you are in the UK) as there is no active growth on them at the moment. if you wait about a month - 6 weeks they should start to spring into action, and then i would start them in water on your window sil, or wait a bit longer if you want to put them straight into soil in the greenhouse. - i have a cold greenhouse and mine did'nt do to well in there during the winter, but the ones that i took in late summer have been fine in my porch, which is fully enclosed and very sunny, but not heated so very simular conditions to my cold greenhouse, but as they had the chance to get started before the cold weather they are fine, - in fact some are flowering now!"