# 1 thing to do is either drill a hole 6" up from the bottom and install a long bolt and nut or deform the post so it will not spin in the set cement. Quite a few threads on this site discuss ground pole installs.
1. Some recommend the fence post cement mix, but I haven't noticed a difference.
2a. Depending on your location and the type of soil, you will need at least a few feet of post below the frost line. I usually dig at least 2.5 foot deep and dig/pour in a tear drop shape, slightly larger at the bottom.
If you are in the north, do not pour a top slab that will be moved via frost heave. In the south you may need a slab to stabilize and "float".
2b. Tools are sold to make jobs easier. Have dug plenty of holes with only a shovel and often needed a breaker bar.
3. You don't want to be moving the pole once the cement starts to harden. This will make the pole wobbly. If the cement mix is not sloppy, I usually set, plumb and level the pole then walk away. If you need extra assurance, wire or rope stays attached from the pole to stakes. No need to keep checking... Do it right once then let it cure. Let it set a day or longer to cure. I have rushed the process successfully and other times paid dearly for not being patient!