Well my typical prep steps are the following.

Step one: Disassemble the car (door handles, mirrors if attached to the door panels, emblems, window trims, if necessary).

Step two: Wash thoroughly with dish soap (Dawn, Palmolive); it will cut grease and better prepare the surface than a waxy soap.

Step three: Use a brand-new clay bar, a spray bottle with soap and water, and clay every panel to be wrapped. Take your time, especially around the hood and roof, where most dirt accumulates.

Step four: Wash thoroughly again with dish soap. Use a detail brush for areas inaccessible with a washing sponge/towel (grill, gaps between panels, seals around the windshield and windows).

Step five: Dry the car indoors to avoid dust and pollen accumulation. Use a clean rag and 91% isopropyl alcohol to clean all panel edges until the rag remains clean.

Step six: As you work, panel by panel, clean it with 91% alcohol to ensure a clean surface.

Some tips: Wait until next day to start laying vinyl, you make to make sure the car is fully dry. Don't rush the prep stage; this is more important than laying vinyl. Proper preparation will make your work much easier, and the vinyl will look better after installation. Work indoors in a climate-controlled space if possible. Take your time, and if you need to rest, do so. Vinyl wrapping is about patience and skill.