When you pick an area, click the R3 button as
@syntex123 mentioned. Anywhere in blue you can place a decal. It will cut off going onto the white. If you can't see the decal as a solid as you're moving it around and it only flashes, it will not be visible when you save. You can use this to your advantage on small areas like the mirrors where you can but a huge square decal to change the color of the whole mirror.
If you need to have, for example, stripes down the hood and onto the front bumper or fenders you will need one on the hood and another on the body section lined up, which can be a pain for some decals where you might do a lot of custom scaling but work at it like any other skill and you'll be able to get it right in short order.
FYI, I find the "align to camera" feature to be very handy when trying to line up 2 decals. Stripes down the length of a car body in particular have a nasty tendency to bend in some way and throw off alignment.