You may want to re-mask everything, because dust and paint on the used masking paper can find its way onto your new finish. And remember to clean off all dust on the car by hitting it with compressed air or using tack cloth.
For rolling on paint:
Get a foam paint roller - 4" wide should do, and make sure it's as fine as possible. This creates a very smooth finish if the paint is thin enough.
Mix acetone into the rustoleum in a mixing can. I've read that you want something around the consistency of water, which means a LOT of acetone. You'll probably need more than 1 quart to do the whole car. When mixing paint, stir it with a stick, DON'T shake it or bubbles will happen.
Note: This method requires a lot more patience than spraying, as you're supposed to do 8 or 10 coats, sanding in between each one if orange peel starts happening. I highly reccomend you read the original source of this method (which inspired this entire project) here:
For spraying it on:
To spray on paint, mix a little acetone into the paint. The can recommends no more than 5%, but don't worry about that since the thinner the paint, the smoother it goes on. However, it is also more likely to run on vertical surfaces so be careful.
This process is somewhat risky, but has great potential. Hard to get areas like door jams, cracks, etc will look amazing when the paint is sprayed on. On the other hand, the entire car may turn out looking like an orange. If that happens, you probably need to mix in more acetone.
If you get lots of orange peel, fish eyes, or other demonic paint problems, you can always sand them away and try again, and in hard to get to areas it won't matter anyway. Spraying on multiple coats also makes for a smoother finish. Wait a few hours between coats to allow drying.
Leave the paint to dry peacefully for at least a few days. I let my car sit in a dry garage for over a week before putting any of the trim back.