This website is here to provide you with the
necessities to 
✿ Owner: Kayla border design: corner pixel
I am assuming you know how to make a border psd using fill layers (or layers I guess)You'll be making something like:
The Steps
1. Define this brush.
2. Select the color white. Use the pencil tool to place the brush in the corner. ZOOM IN to make sure it's lined up perfectly!
3. Add the grey blocks. If you don't know how than -> {CLICK HERE}
4. Duplicate the layers. Select all duplicated layers. Click "Edit >> Transform >> Flip Horizontal." It should now be flipped to where you can put it in your right corner. Use the move tool to move it.
5. Duplicate ALL of the layers. Click "Edit >> Transform >> Flip Horizontal." Use the move tool to move it down!
Finished: