Mitering the edges of two panels that but up against each other is the best way to create a stone facade appear seamless.
Here's a quick tutorial on how to cut corners that you can be proud of!

To miter a panel on your own you will need:
Measuring tape
A Power Drill
Caulk (preferably a color that will blend with your panel)
ATable Saw or Circular Saw
A File
Touch-up Paint

Step 1:

Measure and mark the end of your panel. Remember to measure twice, maybe three times, and cut once.

Step 2:

With your saw set at a 45 degree angle cut the vertical length of the panel. This will be the piece hanging over the preexisting corner to meet your other panel.

Step 3:

Flip the remaining panel over and cut a 45 degree angle that is reversed to the last cut. This should leave you will a small triangular scrap with as little of the face of the panel on it as possible.

Step 4:

Install your panels to the wall, making sure the miter is nice and tight and the edges of the panels are aligned. Parts may of the miter may overlap but can be adjusted.