You can easily attain this after getting your first safehouse (Shaundi's Apartment) unlocked not long after completing the second mission (which unlocks The Welcome Wagon in Saints Row: The Third).
I found the easiest method was to steal cars from outside one of your cribs/safehouses and take them into your garage and customise from there and I suggest using the quick hijacks
) to steal the cars to save some time and to work towards Bo-Duke-En in Saints Row: The Third
Once you've hijacked a car drive into your cribs garage and into the store area and press
to store the car. Now walk up to the area again and press
to open the garage menu, now select which car you want to customise and press the Customise
You can pick anything you want to change
as long as you change at least one thing on the car for it to count.
Some of the body mods have an option for None
which is a free customisation but i found it faster to head into Colour
options and change one of the car's colours which costs $50 (Be a total of $500 for the 10 cars) but as you received well over $4000 from the first few missions you have plenty of cash to customise the 10 cars.
Accept the customisation and exit the cars menu and thats your 1st 'pimped' ride, repeat nine more times for the trophy to pop.
Here's a video demonstration.