Jailbroken Ipod/Iphone/Ipad (Google for jailbreak, i used my iPhone 4g for this method)
Flex (Found on cydia for free, DON'T BUY THE 4$ ONE, KEEP READING FOR THE "FREE" WAY OF DOING THE GLITCH, I'M SAVING YOU 4$)
Computer
Ipod/Iphone/Ipad USB Cable
Download "IFunBox)
brain

I include everything you need possible to do this "Glitch" which is sort of more like a hack.

1) Download on your iFunBox on your computer (So you can transfer files from Computer to iDevice
2) Connect your iDevice and open iFunBox
3) On the top left of iFunBox you should see iFunBox Classic, Click that and you should see you iDevice 4) Go to Raw System Files/Library/Application Support/ Then you should see Flex
5) Copy the Downloaded patch there and replace yours if you have one,
6) Then Respring or Reboot your iDevice. I know it might sound like a lot but its fast and easy

link for patch [ Register or Signin to view external links. ]

With iFunBox make sure to delete the file called "Patches" before inserting this one, so once you open up "Flex" again you'll have all my patches, which include a few good ones :p

After getting the patches, Download Ifruit for your device, once this is done. Continue with the rest.

Go to settings>Online>Options>set it to spawn at last location. Than continue.

1) Go to GTA Online invite only session
2) Get in the car you want to upgrade
3) Go to the GTA iFruit app/Los Santos Customs/ All the way down select your vehicle
4) Go to Story Mode
5) Choose the parts you want and click on "Order"
6) Press start, go down to "Join GTA online"
7) Start an Invite only session
8) Done, your car should have all the customizations possible.