I will make the assumption you want to do this while the engine is in the vehicle. This is a good idea, helps your engine cool off more efficiently and you will get better gas mileage and possibly a few more horsepower. First off, disconnect your battery negative terminal first then cover your alternator in plastic and tape. Make sure your alternator is completely sealed in. Use engine degreaser available at most part stores ) to spray the block and engine components. Read the instructions and warnings on the degreaser, some are bad for rubber parts ( such as fuel lines, electrical wire insulation and spark plug wire insulation ) and take the necessary precautions. Let the degreaser do it's job by waiting the prescribed length of time listed on the degreaser can. Then hose off the block. You will probably have to use some rags and elbow grease to get off all of the gunk. Dispose of the refuse in a legal manner and once your motor is completely dry, take off the cover on your alternator and reconnect your battery.for more info about auto repair for cleaning engine you can look more on the site.