I tend to put in web search the item I need and look at images. I then get a large piece of paper and start sketching to the size I would like. I look at drawings normally as these are easier to follow than actual pictures. I draw in all my detail onto this paper. I then cut out the picture to make the pattern. Think dress making. I then pin this to the design board and using a pencil I scrib into the oasis, then remove the template and continue from there. Yon can buld up the oasis where needed. I find this the easiest way to do any of these designs. I have some in my gallery. I did a double decker bus a while ago using the same principle. If the customer has a particular colour scheme for the bumper, go with this.
Hope that helps.
Treen has some wonderful stuff on her site as well.