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Different flashes will have different brightnesses, so you don't want to fix the complete exposure across all the pictures.

You should fix the ISO to a good value for your camera.  Set it to 400 if you're not sure what the best is.  That should be pretty good with most cameras, and the important part is that it's the same across all the pictures.

The shutter speed will automatically go to the X-sync speed when the camera knows it's using a flash, unless you have a very old or unusual camera.  Still, manually setting the shutter to the X-sync speed can't hurt.

The other part you should fix is the scene itself.  Mount the camera on a tripod and point it at something that will more easily let you see what the flashes can do.  Then don't vary the camera position or the scene it's looking at.

That leaves the last bits of exposure, which are the f-stop and flash brightness.  What to do here depends on how much the flashes vary in brightness between each other, and how much automatic interaction there is between the camera and the flash.

For a "dumb" flash (always produces the same amount of light), do the math and set the f-stop accordingly.  I'm not going to go into details here because there should be much already out there.  Briefly, you look up the "guide number" for your flash, convert that to the ISO you are actually using, measure the distance to the scene, the use the distance and the guide number to compute the f-stop.

Some flashes, particularly those from the same manufacturer as the camera, can be controlled from the camera.  The setup needs to be so that there is enough light as above, but you can set things so that the picture would otherwise be over-exposed.  The camera and/or the flash measure the light coming back from the scene, and stop the flash when enough light has accumulated.  The flash duration is automatically adjusted to get the right exposure.  If your flashes are all of this kind, then set the f-stop a bit wider (lower number) then required, and let the camera/flash adjust the effective flash brightness on the fly.

In the end, though, you probably won't get good results without actually <i>understanding</i> what is going on.  It's time to not be a noob anymore.  Sit down and learn what is really happening.