Unless you have a LOT of posts, the best bet for me has been to use a
product called "Sakrete" concrete mix. This is a pre-mix of Portland
cement, sand, and gravel. Come in 80# bags with mixing instructions
(how much water, etc.).

I like to mix things up in a old empty 5 gallon taping bucket, or
something like that. Easy enough to handle, but having a wheel barrow
to move things around on is especially helpful. Mix as per instructions
on the bag, pour in hole around post, true the post up, and that is it.

Available at most good hardware stores, Home Depot, Lowes, etc.