I make all my decals myself and find it very satisfying.
Use the highest resolution that your printer can do. Anything less than 600 dots per inch risks getting a ‘grainy’ edge, especially on small writing.
Unless you have a fancy printer that will print white, try taking a photo of your car and then scanning that colour to make a matching background for white decals on white decal paper,
Use Johnsons ‘Klear’ floor polish over the decals a day or so after you’ve applied them. This ‘sets’ them in place and stops any lacquer/varnish from making them wrinkle.
Good luck!
Mike