First open photoshop and then file new dont matter on teh size.
I will be working with 700x100.
Make background any color u want. I made mine black.
Now put ur renders or anything u want i will be just brush strocking.
Now time to get the lasso out.
Draw on ur image kinda like mine (image below).

U must still have the lasso out. and right click the inside of ur lasso.
Then click stroke. put the color on grey
then change the opacity to about 15-50% (i put mine 30%)
now u got a nice shadow.





If u dont like the tutorial please dont say anything like i said i made on request u may use it if u want.