Hopefully my bad luck has passed now and I can get this posted.
My attempt...........
First Jpeg final results and pdf file to follow on drop box
Drop box file
https://www.dropbox.com/s/iagshnedzd3ea5g/Altered atardecer en el arenal (5).psd?dl=0
I didn't have to do much all it really needed was select and mask, feathering and brush to get rid of sharp edges and marry the soil to the subject. I took screen shots as I went so the my words don't get lost in translation to Spanish.
First I turned off the eye ball to each layer and got down to the First Background background layer you marked as layer 0
Then I duplicated that layer and then sharpening to the back ground only
Turned on layer 1
Used the Quick Select and outlined the subject which was the tent, then pressing Select and Mask at the top I brought it in and using the two buttons at the top + - I fine tuned the image brushing in mask in red so nothing was left but the tent itself, making sure I feathered the image at 1.6 so I had no sharpe looking edges. Press ok
Back in PS I deselected the marching ants around the tent then pressed CMD T which allowed me to move the tent around, then right click within the box you can scale, warp, distort etc.......Using the warp I dragged the corners down just a bit till it looked natural, then pressed the check mark at the top when I was satisfied.

Now to merge ground with subject......To that layer apply the mask, which is the third icon at the bottom ( white square with a black hole in it). This bring up a box where you will mask out what you dont want and fine tune the merge. Click on that empty box, Press B for brush (use a soft brush ) Press D for default color of Black and white.
Lower your opacity of the image down till you can see through it and using the brush use black to bring up the grasses and sand around the bottom of the tent, if you make a mistake switch to white and it will bring it back to start over. Go around any sharpe edges with black lightly to rid any sharpe edges showing.
lowered opacity
Painted in black on the mask only
Bring back the opacity up after your done for final view and continue brushing if you see anything you dont like.
Turn on the next eye ball to the next layer as you progress up
Part two to follow next