Lowering a car, is an integral part of Chopping. It is almost always the first step, it helps decide the line for the rest of your design and can limit your choice of wheels and other accesories.
The first step, is to choose a car you want to work with
With the polygon lasso tool(or pen tool), and select the wheel arches and base of your car. You want to select all the will want to select all the way around the outside of the image, just to help blend the lowered car in. The red line shows where you should select.
Your final selection should look something like this, and remember... Don't be lazy around the arches!
TIP: When u want to get a nice curve... make alot of cuts (a lot of small straight lines is nicer than a few big ones).
Once you have your selection made, press ctrl + J to make a copy, to make a copy of your selected area. Using the transform tool(ctrl + t) and move it down with the arrow keys and stop at however high u want your car to be (if its too low u will need a shadow on the wheel)
ta-da! Now you have a lowered car! In some cases you may want to flatten the image before you continue on, though you are more likely to keep your layers until after you have put in your final wheels, or even just group them together to keep it neat(you never know when you will want to change something!)
Now with the polygon lasso(or pen tool) select behind each wheel, where there is now empty space.
and fill it with black
and you're done!
You can find the original tutorial written by Mathew Law and many more in the Chop School