Mistakes are the best way to learn. ;-)
Don't get hung up on following just one tutorial either. Every one will have a different work flow, mixing/matching what works for you helps develop your own style also.
A couple things I've learned from various tutorials that work best for me -
-Start at the wheel wells with a circle and delete the extra points. Duplicate the remaining arc and scale up to make the top of the lip then use loft/ruled surface.
-Don't try to make separate panels until you get to the detail stage but do have a good edge following the panel edge. In the blender tutorial he made a small loop between panels and deleted it, that's not really necessary. If you just extract the panel you'll end up with a gap when you subdivide it and it will be uniform all the way around the panel.
-There's almost always a better way to do something, but that applies to just about everything. :lol:
-I left the door out of my car to separate the front and back and make it easy to hide one half wile working on the other. The geometry does line up however and it's just a matter of bridging the two halves together and adding some vertical cuts to fill it in.
- nice flow on the test model.