Bezel setting a stone with sharp corners is somewhat more difficult than an oval because if it isn't done properly the excess metal puckers at the corners.
Bezel setting is often called a "rub" over setting. This is because some people use a burnisher to "rub" the bezel "over" the stone. It is quite difficult to do this because you can't get enough pressure especially in the corners to fully push the bezel over the stone. Because the surface of the burnisher is polished it also makes it more difficult to control the burnisher.
There is a bezel pusher that has a round wood ball handle with a small square metal rod to push the bezel over. It has such a small surface that it introduces dents and creases in the bezel.
My favorite tool is a bezel rocker. It is a wood handle with a metal blade on the end that you "rock" the bezel against the stone.
This photo shows the one that does the best job for me. Notice how the length fits my hand with the end of the handle relatively flat (the edges are rounded, though) and fitting in the heel of my hand. It allows me to comfortably get significant pressure to push rather firmly on the bezel.
Here you can see how it fits my hand when I'm gripping it.
Here are some pictures of other bezel rockers.
This one is too short for me and the end of the handle is small and sharp. It would be rather painful to push against the bezel with much force.
This one is a better length but the blade has far too much dome across the top. It doesn't have enough surface in contact with the bezel to get a good push.
On this one the blade is too thin so it also has too little surface in contact with the bezel.
Here is a photo of the best blade shape on the top. It is about 1/8" thick. If you have a rocker with the correct thickness but too high of a dome it can be ground down to a better dome.
The face of the blade must be flat, be roughed slightly to keep it from sliding off the bezel and have square but not sharp edges so is can be positioned on the top edge of the bezel without slipping off.
The stone must fit easily into the setting but it must be tight enough so that the stone doesn't slop around.
The height of the bezel must be high enough to reach above the bevel on the stone so it can bend over the stone. If it is too high you can flip the setting over and sand the bezel down on a flat piece of sand paper. You can also lift the stone by cutting flat pieces of plastic can lids and placing them under the stone.
A bezel that reaches much above the bevel of the stone will have the excess metal pucker when it is pushed over. It will also cover much of the beauty of the stone.
We will now show the setting steps.
Start by placing the stone in the bezel and placing it on your bench pin. The height of the bench pin must be very slightly below the vise so that when you push on the setting it will have something to keep it from sliding away from you. Push the center of the bezel on one side in slightly with the bezel rocker at a 45 degree angle.
Now rotate the piece 180 degrees and do the same to this side. This is to center the stone in the setting.
Do the same on the ends.
Next we will start the pushing the bezel over the stone. Start at one corner and push the metal AWAY from the corner.
Now go to the opposit side of the same corner and push the metal AWAY from the corner.
The corner should now look like this.
File the top of this corner down slightly at a 45 degree angle.
Now go to the opposite corner and repeat these steps.
Do the same to the other two corners.
You are now ready to start pushing the centers of the sides of the bezel. Push each side part way over with the bezel rocker at a 45 degree angle for the length between the corner. Go to the opposite side and do the same. Now go to the ends and do the same.
Now that the bezel is part way over go back to the corners and start the same steps over with the bezel rocker at an 80 degree angle. Make sure you push the corners AWAY towards the center of the sides. Do these steps in exactly the same sequence as the first time. You will be pushing somewhat harder and at a much more downward angle that the first time.
Now feel of the top edge of the bezel. Your finger should "catch" on the upper, outer edge of the bezel. This is the remnants of the sharp outer edge of the bezel. The inner edge should be tight against the stone.
Your final steps with the bezel rocker are to deform this sharp edge against the stone. You do this by hooking the edge of the rocker on this sharp edge, again starting at the corners and pushing AWAY from the corners, and pushing straight down on the bezel. Here you can see why the top surface of the bezel rocker blade must be square (but not sharp) so that it will catch the top of the bezel.
Again feel the top edge of the bezel. Now you shouldn't be able to "catch" on that edge.
Your final steps are to polish the bezel and corners down smooth.
The final appearance of the piece. Notice that the corners are firmly against the stone, there is no gap and no pucker.