First time babysitting!?!?

i am babysitting for the first time this weekend! i am kinda nervous......but not really! the family im babysitting for has three kids! two boys and a girl. im concerned mostly about the girl because she is mostly potty trained but still wears a diaper just incase......yes she still has accidents! she tells when she has to go. so thats a good thing my problem is.....how do you change a diaper on a 2-3 year old!?! should they sit or stand? kinda funny question but i bet some of you have been in this situation!