How do I add concrete to fence posts?

Dig around and down the fence posts as close to the bottom of the posts. Pour concrete into the holes a few inches below ground level. Cover with soil when hard.

I used to dig the hole, add some rocks/gravel to the bottom for drainage and height adjustment. The post needs to be plumbed and support with braces to keep it plumb. Then I poured water into the hole and then added dry concrete mix. Agitate it with a stick to help mix.

What type of fence and how high? Why bother with concrete? Concrete is subject to the same ground heaving as anything else and may even be more pronounced because the concrete is so rigid and can't absorb any movement. if not done correctly, concrete will accelerate rot. Part of the problem might be your posts are rotted making them thinner and loose, or they are not sunk deep enough for the size and type of fence you have. Adding concrete now will be a temporary fix , but you are creating a major headache when you need to get the post out. I have erected several types/sizes of fences and have always used cedar for posts and used a mixture of soil and gravel tamped to hold the posts (a little landscape fabric and gravel at the bottom for drainage). The only time I have used concrete is for a chainlink fence because the posts are such a small diameter.

brace your posts once it is in the hole, then pour the quickcrete or fast concrete mix in - I mix before putting into the hole

remove from fence, pull up, add cement, place back in the hole.