First decide whos vehicles you want to detail.
Dealers? Soccer moms? people that make $100,000 + a year? classic cars??
Each group has specific needs and purchases detailing for different reasons.
So first pick the group or combination of groups and target them.
Want dealers - Go to them and promote that you can reconditon their vehicles quickly and at a low price.
Want soccer moms - promote at day cares grocery stores and such. Tell them you have a powerful hot water extractor that will clean up all the mess kids do to their SUV/MINI VANS.
$100,000 crowd - promote at high end golf courses, spas, join a business association.
Make sure you pick only one or two groups (if they are simiular) because doing a hack job on a dealer car then a show prep on a classic car is going to take different types of products and techniques.
You ge the idea.