Easy "Los Santos Customs" achievement

First, successfully complete the "Father/Son" story mission to unlock Los Santos Customs (car repair shop). You will also need quite a bit of money to fully upgrade a vehicle. Additionally, not all vehicles can be modified. Open the internet browser on your in-game cell phone, and go to "southernsanandreassuperautos.com". Buy the Weeny Issi for $18,000, and retrieve it from your garage. Then, go to Los Santos Customs, and buy the following upgrades:
1. Armor - Armor Upgrade 20%
2. Brakes - Street Brakes
3. Brakes - Sport Brakes
4. Bumpers - Custom Front Splitter
5. Bumpers - Custom Rear Bumper
6. Engine - EMS Level 1, Level 2, Level 3
7. Exhaust - Dual Exit Exhaust
8. Hood - Vented Hood
9. Horn - Truck Horn
10. Lights - Xenon Lights
11. Plate - Blue on White 2 (buy what you do not have)
12. Respray - Primary Color - Chrome
13. Repsray - Secondary Color - Chrome
14. Skirts - Custom Skirts
15. Suspension - Lowered Suspension
16. Transmission - Street Transmission
17. Turbo - Turbo Tuning
18. Wheels - Wheel Type - SUV - Benefactor
19. Wheels - Wheel Color - Black
20. Wheels - Wheel Accessories - Custome Tires
21. Wheels - Wheel Accessories - Bulletproof Tires
22. Wheels - Wheel Accessories - Black Tire Smoke
23. Windows - Light Smoke
Once you have all of those items, leave the shop to get the "Los Santos Customs" achievement, as it will not unlock while you are in the shop.