STEP 1:

Choose a landscape image. The better the landscape image the better your inception image will turn out. Try to pick an image that has a lot of form (depth). 

STEP 2:

Right click on your background layer and select "Duplicate layer". 

STEP 3:

Press CTRL + T.

Right Click on the image and select "Flip Horizontal"

STEP 4:

After flipping it horizontally, go to the top of the screen and in the angle box change to 90 degrees. Your image should now look like the one on the left.

STEP 5:

Align the bottom corner and left side of the images

STEP 6:

Use the poly lasso tool to create a triangle from the bottom corner to the top point where the two images meet. Make sure you are on the right layer and delete the selection.

See the image below it should align on the right side.

STEP 7:

Now we will begin on the left side. go back to your layer 1 original image. right click it and select "duplicate layer"

on the new layer press CTRL + T then right click on the image and select "Flip Horizontal"

STEP 8:

On the top set the angle to -90 degrees. IMPORTANT- you must put a - sign in front of 90 or it will not work!

Align the image in the right corner and match to the side similar to the left side.

STEP 9:

Using the poly lasso tool make a selection from the bottom right corner to the corner where the images meet.

delete the selection.

you should now see a nearly complete image.

STEP 10:

Cleanup. Use the Eraser tool on the two corner layers to remove any weird looking parts!