Setting fence posts starts with the corners. Once the corners are set, get a mason line and a level. That makes it fairly straightforward to get all other posts perfectly lined up, plumb and at the same elevation.
When a post was positioned, we braced it and temporarily connected it with two by fours to the adjacent posts so that it stayed in place. After a full line of posts was set, we carefully poured the concrete footing around each of the post bases. We knew that if we didn’t do it carefully, we might have moved post base and would have had to start re-aligning. And that is a time suck, believe me!
There is a simple trick on how to make pressure treated posts last. Don’t set them on concrete, set them in concrete.
If you set them on concrete, water has difficulty getting out and the post base becomes soggy which accelerates rot, even if they are pressure treated. If you make sure that the bottom one inch is sitting in soil – or even better, on pea gravel – the post base has a chance to drain and dry out, which should make it last longer.