Dig a hole 18 inches deep and 6 inches wide with a post hole digger.
Shovel gravel into the hole to a depth of 6 inches.
Pour a small amount of cement into the wheelbarrow. Add water and stir with the shovel to make the concrete workable. Shovel the wet concrete into the hole to a depth of 6 inches.
Plunge the post into the hole forcefully. Twist and push on the post to penetrate into the wet concrete until the post bottoms out.
Level the post by holding the level vertically against the post from both sides. Use the shovel to add more wet dirt to the hole as you continue to check your vertical alignment with the level. Finish by filling the hole to the top with wet soil. Tap down the soil with the end of the shovel. Check the vertical alignment again and straighten if needed. Wait 24 hours for the concrete to harden.