If you pour concrete, and insert the post while the concrete is still loose, it will fill the post to the level of the concrete around the post.
If you want to fill the post the rest of the way, simply thin the balance or the left over concrete to a slurry that you can pour into the post from a bucket.
Make sure you have the post well supported, an plumbed, and squared with the opposing post.
I recommend you use a step ladder, lol.