If you want to pour the concrete footings then come back to put up the fencing, do what the locals do and pour the concrete with an upside down plastic water bottle in it. You can then come back, cut the ends off them and drop the post into the bottle and secure it with a splodge of cement.

Helps divide the task into the messy and the neat stages.

PB
(You may have to fill the water bottle to stop it crushing)