It's good to take your time to begin with

If you've been using the pen tool here then well done! Your selections look great and super tidy

Keep practising with the tool, and it will get easier and easier.
When it comes to using levels, just be careful that you don't make the change too extreme, it is perhaps a little bit too dark this time, but hey - you're learning
Here's some important skills you may want to try on future chops:
>Wheel change - wheels make all the difference to a car, and it's fairly easy to change them too, it is also possible to make mistakes - just make sure your donor image is the same angle as your base image, otherwise your wheels won't look right
>A backdrop change - if you pick the right base, it can be very easy to change a backdrop, again just make sure it's at the same angle. If you have quite a plain base, you can sometimes just change what is above the horizon with the same effect - that way you don't have to worry about shadows on the ground. Check the 'Tutorial Index' in the tutorial section if you need some help there
Keep it up though - slow and steady is a good way to go
