Simply looking at another vehicles meta files will be a massive help.

In short, for a working modkit, you need to define a modkit name in the carvariations.meta, which must be matched in the carcols.meta, and a valid modkit ID (<1024) used.

From there its just a case of adding the parts as xml <Item>'s, again, you're best looking at an existing vehicle to get an idea of how it works.