-At first, open the Photoshop.
-Click to file menu.
-Click to open.
Open/insert a fat-man’s picture.
Click to the Filter menu.
Click to the Forward warp tool from left side
Change the brush size to 200, brush density to 17 and brush pressure to 93.
Repeatedly, click to the white area and drag right to left side as picture below.
Drag from left to right as long as the belly will smaller.
Accordingly, try to slim to the hand and others arms that need to slim.
Then Save the project as a picture.