There are as many ways to repair wooden fences and fences as there are nails in the fence. The way I describe here has worked for me here in the Dallas Texas metro area for many years.
Do it yourself fence repair: Is easy if you do it right but is very difficult if you do it wrong and can be very frustrating and costly.
Replacing rotten fence posts: Replacing posts in a fence is one of the most difficult things about repairing fences. I’ve seen DIYers try everything to get a broken fence post out of the ground. One of my favorites is what I call the Grand Canyon. It’s when a do-it-yourselfer digs a hole so big around the fence post that they almost need a cement truck to get enough concrete in to fill it.
Have you ever dug a hole for a fence post? If so use the concept of digging an 8 inch hole for a fence post against the concrete of the old fence post about 2-2 1/2 feet deep. Next, take a sharp shovel to clear some dirt from each side of the concrete. Use the post hole digger to remove the small dirt that you loosened from the post hole. You now have a hole deep enough that with a little effort you can use a rock bar to lift the broken post and concrete into the hole you just dug so it will be easy to lift out.
Put the new post in the hole, take the old hard concrete and use as filler in the hole and put as much premixed wet concrete in the hole as needed to fill to the ground level and then seal the post with a level. You can then wait 24 hours for the concrete to harden around the new fence post, nail the fence panels or you can nail the old or new fence boards to it, level the post and then use an old fence board to support it.
If you want the easier way out, you can install a new fence post next to the existing one so you don’t have to dig out the wood post.
Cedar Fence Post: Cedar is naturally very resistant to rot, decay, warping and insects when used above ground. If it is saturated with moisture when it is installed in the hole with concrete around it and dries out, it will shrink and leave a void that will take water. This creates a premature decay process. This can also happen at ground level if the concrete is not poured to a level that helps repel the water from the fence post. You can use a good weather treatment to soak the post before installing to extend like a cedar fence post. I have used Behr and Olympic with good success.