So the ol pressure treated wooden post in a concrete footing.
Removable post in concrete.
The concrete should fill about to of the barrel to provide enough stability for the posts.
This pipe must remain empty of concrete with the bottom open to the ground below.
Check if your post is plumb using a level.
After the concrete is set 2 of coarse gravel is added inside the pipe to provide drainage.
Move and adjust the post if you need to until it sits plumb.
Once your concrete is in the hole use your two sided post level to make sure your post is straight.
Make sure this pipe is large enough to hold the removable post and 18 24 deep for stability.
We filled the barrel with about 8 10 of concrete to a make a 16 18 planter.
Set planter barrel in place and mix the concrete in the barrel.