Shovel three loads of gravel into the post holes or until the hole is two-thirds full. Load the post into the hole and shovel the gravel around it.
Shovel the concrete around the post while keeping the post close to plumb using the level. To use the level, locate the two tubes (one horizontal and one vertical) on the body of the level. Place it on the top of the pole so the horizontal tube is parallel to ground and the bubble centers itself between the two lines on the tube.
Set the level vertically against the pole and ensure the bubble in the vertical tube lines up in the center.
Continue to shovel in concrete around the post until the hole is nearly filled. Cover the last inch or so with dirt to secure the post.
Adjust the post with the level and prop it up with wood stakes if the pole wobbles or is tilted in any way. Allow the stakes to hold until the concert dries completely.