Study the vehicle wiring diagrams side-by-side with the installation manual for the intended car accessory. If you do not have one in hand, visit the Internet to find what you need. In most cases you get the information you need for free regardless of the type of vehicle you own.
Locate the car fuse box since all electrical components of the car are connected to it. The fuse box is composed of slots for each component already installed along with extra ones in case you wish to add more. The car's fuse box for most vehicles is located behind the dashboard, below the steering shaft or inside the glove compartment.
Find a suitable position and mount the accessory using screws or reinforcement braces that come with the package. Note down the amperage for each accessory before installing it. Use the accessory at amperage not exceeding its own. Confirm that the accessory's cable comes with a connector fitting perfectly into a slot on the fuse box. If so, your work reduces. Otherwise, you strip the tips on the wires of the accessories. Insert them into the slots then tighten the screws holding the wire firmly in the slots.
Confirm that each cable joins to a slot directed by the installation manuals. Pay close attention to the wire color coding for each component you install. A misconnection might cause a short circuit or damage the components involved.
Replace the dashboard or any other covers you opened to expose the fuse box, then start your vehicle to test your accessories.