A video I made whilst we were putting Postsaver protective sleeves on part of the stock of fence posts.
It is a really excellent product, which should more or less double the length of life of a fence post, and takes little more than a minute to apply once you are set up.
But make sure to buy direct from the manufacturer, because retail outlets will gouge you comparatively. The starter kits are particularly good value.
And they do trade accounts if you have a repeated need.
The feature photo repeated below shows an alternative way to create a long-life fence - use a repair spur and keep the wood off the ground completely.