Digging holes two feet deep is hard work. There will be roots and stones. It is especially difficult if there is an old lump of concrete in the ground for some old wooden post that has rotted away, and professionals will leave it and dig new holes between the old lumps to avoid this work. Fences that were put up by builders are particularly prone to having large lumps of concrete, but often too shallow. This is so hard that I have resolved never to use wooden posts again, only concrete. Wooden posts rot and beak off at ground level, where the moisture and air content are optimum.
A concrete post is very heavy, though two sturdy people can manage it. Same with concrete gravel boards, which I also recommend for the same reason.
It is kind to leave a hedgehog gap, perhaps at the end of the run, or leaving out one of the gravel boards .
You can get a special spirit level attachment that straps onto the post. As a beginner, put the first one in, get it perfectly upright, wedge it with bits of brick or concrete while you shovel in the mix, ensure it stays perfectly upright and nothing happens to move it for at least a day. To prevent a large expanse of lumpy concrete showing above the ground, you can make a simple shuttering box around the base of the post at ground level so that the top few inches are small and neat. You can then dig the next hole, and by the time you have done that, carried the post out and had a tea break, the concrete in the first hole will be stiffening, though it will have no strength, so you might like to wait until the next day before you position the gravel board and fence panel and position the next post against them (not so tight that you can't slide a new one in from the top when required).
If you are an amateur I would be quite happy to do only one post a day so you can be confident you will not loosen the previous post in its concrete before it has time to harden. Otherwise, do them quickly enough that the concrete is still fluid and you can tamp it down again if it moves. On a windy day the panels will rock the posts and loosen them in soft concrete. If you are working fast, you need each hole to be ready before you start fitting the preceding post. This will need about three people. A straight-sided hole the shape and width of a bucket is better than a great wide crater.