So here's the short version...

You need the Vehicle Build/Repair skill to build and repair vehicles. You also need Car Tools or a Car Shop. The Car Shop will give you a paragraph discount.

Do a certain number of paragraphs, based on the table, per 10% TRP.

People can work together on vehicles. You get a 25% discount of paragraphs of RP if you have a second person.

For modification or detailing, do 10 paragraphs for every $100 that the modification is.

For more details and reasonings, please read the longer version of the rules.