If you're going full DIY and you've got access to Photoshop (or similar) then you can design whatever you want. Just hunt around for "blanks" or templates that have the outline of whatever car you're applying it to. You may have to pay a nominal amount for one but it's a good investment, especially if you enjoy the design work and want to offer it to others as a way to earn a bit of extra cash.
Once you're done with the design, save it as a vector file and then the hard part: finding someone with a vinyl cutter. Most vinyl cutters also do design work, and would charge for both, so if you approach them just want the vinyl & cutting done then it may be more expensive as they're missing out on the design work revenue (which is normally the bigger of the two). But if you find someone who is solely a vinyl cutter then the price should be better, but not always the case.
Quality of the vinyl is another thing to consider, so try check out some of the vinyl before you choose it. And then it's up to you if you want to apply it. Clean the car 2-3 times, using a microfibre to dry it. Include door jams and the inside of the fender/guard lips (get all the road grime and rubber off). Get each piece of vinyl and tape it onto the car using blue builders tape to rough out where they will all sit. Fill a spray bottle up with 95% water and 5% dishwash detergent and spray the sticky side of the vinyl first, then a light spray on the car. Then apply it and get it in the right position. Use a credit card to squeeze out the suds & bubbles from underneath once it's in the right position, starting from the middle and working out. Use a pin to pop any bubbles that you haven't managed to squeeze out. If you have a heat gun / hair dryer handy then heat up the vinyl slowly and gradually when trying to form it over & around curved surfaces like door handles.
For your first time i would do each piece of the vinyl on one side, then immediately on the other in order to ensure consistency between each side. Keep a sharp razor blade handy for trimming around door handles and door jams. Also keep that microfibre handy and wipe each surface before applying each piece.
The biggest tip is take your time. Don't have a massive dose of caffeine before starting, you want to take it slowly. Ensure the vinyl wraps around the edges of doors and guard lips too otherwise it looks crap