Need to fill a hole 8"x8"x24" deep. There will also be a post 4"x4"
occupying this space. How much concrete do I need? Will a 5 pound bag be
sufficient?

Which type do I need? Regular or quick drying? Is Quikcrete what I need?

I assume I need to put in some gravel, then pour the concrete in, then
insert the 4x4 post and make sure it is level. Now do I pour all the
concrete in all at once? or should this be done in several steps? While the
post is sitting in the wet concrete, do I need to hold it still until the
concrete sets?

Should the post be sitting on top of the gravel then concrete is poured? or
should I insert the post, pour in gravel to hold it straight, then pour the
concrete in?

Is it better to dig a square hole 8x8 or a round hole 8"R for a post
intended for a mailbox?

Thanks,

O