Hi bexta,
If I was you I would,buy a 6 X 3 plasterboard, cut it into pieces and practice on that,You could pin it to an old wall,once the plaster is on and drying,you won't get it off,don't press too hard, take your time, you won't get a perfect finish as soon as you put it on,as it starts to go off you go over it again,if it is going off very quickly in this heat,flick water onto it with a paint brush,
Keep the float spotlessly clean,during and after working with it, the plasterboard will cost about �3.00 for a sheet,
good luck, Ray