You need to place a bunch of 'slots' down to do the work of building a car, in a FIXED order. You connect the output of one slot to the input of the next slot by clicking and dragging the conveyor belt between them
The order of the slots is:
No slot will do anything unless it has the right resources. At the start of the game, you will need to import almost everything. To do this you must place down resources importers on one or more of the available input sockets, and connect them to the stockpiles of each slot using the resource conveyor. The correct resources will then be imported and carried to their destinations automatically.
You can place down research facilities anywhere on the map. When you have one or more, you can use the research screen to research new car features, new production line slots, and other capabilities.
Expanding the line
One of the most likely early research targets will be specializing your production. This unlocks extra slots, allowing you to break down something slow like building the chassis, into a bunch of quicker, shorter steps, making for a much more efficient production line. Once you research a step like this, you may wish to destroy parts of the old line and re-route things. Note that the slots are always listed in the slot picker in the order that these tasks must be carried out.
Expanding the factory
Once you research factory expansion you can simply click on one of the nearby factory lots to expand production directly into this factory space. Factories are rented, not bought outright, meaning that you need to keep an eye on the size of the factory along with the rent per tile that it charges.
If you run out of money...you lose. Don't get too ambitious with your expansion!
Each combination of features on a car is a separate car design for you to sell (and name). The game will give you a rough guess of what you could charge for the car, but you are free to sell at a discount or a premium. premium priced cars will take longer to sell, whereas heavily discounted cars will sell very quickly. As you increase the output rate of cars, you will thus have to adjust your pricing downwards to avoid having too many sat in the showroom unsold.
In a perfect world, a car moves seamlessly from the first second of construction to the export slot without ever pausing, and always being worked on. This is almost impossible, but when it happens, the icon on that car (as you zoom out) will be green. Yellow icons mean a car is delayed, and red means its actively prevented from progressing. Aim to have a nice flowing efficient factory with zero bottlenecks.
Good luck with that :D