It is shown on the How to play section of Main menu.
You just drop things onto eachother after breaking a loot pile.
1) Stop your car (S or Space if you have a handbrake),
2) Get out with F.
3) Walk to the lootpile and press F to "search" the pile.
More items pour out if you do this as a character.
4) Drop the items onto eachother (F) to create a machine.
5) Experiment with parts. They are all just a 2 items combined into one.
Usually it involves a "Casing", but there is one exception.
The car smashes the lootpiles and only limited items fall out then.
There's only like 6 things you can build. If you press K in game, you can see which machines you have installed on your car. They also break fast so you can repair and replace them by crafting new ones. The game is very simplistic though, there's not much to do and the goal is to just drive the farthest distance you can, while managing your car's machinery and ability to throttle.