In this guide, you will get to know how you can customize your client cars in GTA online. You will need to complete one contract before you can start doing these missions. Once you complete both the preps and the final mission, you will receive a message from Sasana in which you will be informed that you are eligible now for this side mission. She will message you when any vehicle is available. You will have a look at this type of scene when any vehicle is available for upgrade.
You just need to walk up to the car and press the d-pad to enter into the upgrade services. This is how the scene will look like.
You will see the certain request of what you need to do to the car at the bottom of the screen.
Once you have completed all the upgrades needed by the client. Then you will need to to put the hidden six wheels on.
After that, you have to click on the Confirm button, you will be paid $20,000 out of your pocket.
The next thing is the delivery of the car to the client. It is recommended that you deliver the car by yourself because in this way you will be able to get more money deposited into your account with less cut on the amount. If a staff member delivers the vehicle, then it will take 15 to 20 minutes for the amount to be deposited into your account.
Therefore, it is recommended to deliver the cars by yourself instead of asking any staff member to deliver them. You can have more than one garage to upgrade more than one car at a time. Getting this contract is completely random as in some cases, you can get the contract immediately or in some cases, just wait for some time by staying in the free roam.