My soil was packed clay so I added a little sand here and there to make the pavers level.

It was fine for a few years, but fire ants have undermined it in places and it is now high in the middle and slanted on the edges.

I must redo it. I have not decided what method to use to prevent it happening again.

I put down about a 2" layer of sand. After 7 years it isnt as flat as I would like, but there has been less freeze/upheaval than I thought there might be. And yeah - ants do love the sand. If I was going with a meandering fieldstone kind of path, as opposed to a more structured symmetrical path, I would forgo the sand next time.

For a nice level, solid path of flat stones or store bought pavers, it's best to dig up the area where you are going to lay the stones to a depth of at least 4". More if your stones are thicker. Make an extra effort to level out the depression, making it as evenly flat as possible. Add a layer of sand 2/3 to the top,and if you can, a layer of washed gravel of say 1.5" BEFORE the sand layer would be great. Put down your stones/pavers and line up their straight sides (try to find stones with at least one good straight side) along the edges of the dug out area. Fill in with stones, laying them as tightly against on another as possible, use the handle of a tool to "hammer" them down and up against one another to make them fit snug. When all the stones are laid, pour more sand over the entire area including over the stones. Then take a broom and sweep the path, making sure sand gets in between the stones. After that, I always take a hose and spray water over the area to settle the sand. Walk over the area and feel for any loose stones, ones that rock back and forth and are not set snug.

This is when it is good to have a pile of small to very small stones of all shapes, flat and not-so-flat. If you have a stone that wobbles when you step on it, dig around it and place these small stones under as "shims" for a better word, to help level up the stepping stone. After all this, you could dump more sand down if you wish and sweep it off and wet down the whole area again.

I think this gives a natural looking path. Yes - you will eventually get weeds growing between the stones, that's what happens with natural stone paths. Don't use mulch as a filler between the stones, it won't deter weed seeds from blowing in and growing ontop of it; and it won't hold your stones or pavers snug. I don't like the look of weeds or any "steppables" growing between my stones so I try to be vigilant in picking them out or using a weed killer.